CNN  — 

Senate elections typically happen only every two years – except sometimes they’re three years in a row.

Democrats picked up two seats in November 2020. They won two more in Georgia runoffs in January 2021. And in 2022, they’ll be fighting to keep control of the evenly divided chamber, where Vice President Kamala Harris is the tie-breaking vote.

But the fight for control is already center stage, since Congress – and the 50-50 Senate, in particular – helps shape how successful President Joe Biden will be in enacting his agenda. Democrats are eager to grow their majority so they can pass legislation with a more comfortable margin, while Republicans want the Senate back so they can check the Biden White House.

History is on Republicans’ side. The party that loses the presidency usually gains seats in the midterm elections. By the numbers, at least, Republicans are more on defense heading into 2022: Of the 34 Senate seats up next year, Republicans are defending 20 to Democrats’ 14. Not all of those are competitive, though: Only eight seats are currently rated as “battlegrounds” by Inside Elections with Nathan L. Gonzales.

The seat most likely to flip partisan control next fall, according to CNN’s inaugural ranking, is in Pennsylvania, currently held by GOP Sen. Pat Toomey, who’s not running for reelection. The top 10 Senate seats most likely to flip are based on CNN’s reporting, as well as historical data about how states and candidates have performed. As the cycle heats up, polling, fundraising and advertising spending data will also become factors.

In this first edition, the top 10 slots happen to be evenly divided between GOP-held and Democratic-held seats. Three open seats are on the list, all currently held by Republicans who are retiring, making their seats more competitive than they otherwise would have been.

Regardless of historical precedent, the map of seats at play is important to consider. In 2018, for example, Democrats won the House by capitalizing on anti-President Donald Trump energy in the suburbs two years after he had been elected. But the Senate was a different story: Republicans gained seats because Democratic senators were up for reelection in rural red states where Trump had won.

This cycle, only one senator is running for reelection in a state carried by the opposite party’s 2020 presidential candidate: GOP Sen. Ron Johnson. His Wisconsin seat is the third most likely to flip, but the two-term senator is the most vulnerable Republican incumbent.

Some states on this list will feel familiar. It wasn’t too long ago that Georgia was the epicenter of the political universe, helping to deliver the White House and the Senate to Democrats. Expect the Peach State to be a big player again in 2022, with Democratic Sen. Raphael Warnock running for a full six-year term and control of the Senate on the line once more. Among the other states that were hotly contested in the 2020 cycle and again are on this list are Arizona, North Carolina and Colorado.

But long before the “Magic Wall” is coded in shades of red and blue, intra-party battles will dominate much of the news in 2021, with next year’s nominating contests going a long way toward determining how competitive some of these general elections will ultimately be. Senate primaries – the fields nascent as they may be – are emerging as early indicators of where each party’s base is headed. That’s especially true for Republicans, who are very publicly grappling with what the GOP looks like with Trump out of the White House.

The former President may have left Washington – and in unorthodox fashion, as the first President to ever be impeached twice – but the control he has over the Republican Party was on display at the annual Conservative Political Action Conference in Orlando this past weekend. It was all about him – gold statue and all. And even if his winning percentage in the unscientific straw poll wasn’t commanding (55%), the rhetoric his acolytes and other 2024 prospects used was a nod to “Trumpism” – a noun that the former President delighted in defining onstage in his first public remarks since leaving the White House.

Listing every Republican who voted to impeach him in the House or convict him in the Senate, Trump is targeting them for removal by challengers more loyal to him. He recently endorsed a former aide who’s running against one of those House Republicans, and as he considers launching a super PAC, he’s signaling he may be investing in additional races with more than just endorsements.

The extent to which Trump will get involved and will back candidates at odds with Senate GOP leadership remains to be seen. But he could be a potent factor in open-seat Senate races, like those in Pennsylvania, Ohio and North Carolina, where GOP incumbents – two of whom voted to convict him – are not seeking reelection.

Sen. Lisa Murkowski of Alaska is the only Republican senator who voted to convict Trump who’s running for reelection next year, but he’d threatened her even before that vote. And the moderate Republican proved in 2010 that she can lose a primary (and GOP leadership’s support) and still win the general election as a write-in candidate. Senate Minority Leader Mitch McConnell pledged on Monday that national Republicans will stand by her. Also working in Murkowski’s favor this cycle may be a new “top four” system in the state, where all candidates run together in a nonpartisan primary and the four top finishers advance to the general election, where voters rank their preferences. For all those reasons, Alaska doesn’t come close to cracking this list of seats most likely to flip, despite Trump’s threats to Murkowski.

A few other GOP-held states could eventually earn honorable mentions. For now, the only chance that Iowa becomes competitive is if the 87-year-old Sen. Chuck Grassley retires. He hasn’t said what he’s doing yet. But even Joni Ernst, the state’s junior GOP senator, who was in a top-targeted seat last year, won reelection by more than 6 points.

Similarly, Missouri looks to have become more solidly red since GOP Sen. Roy Blunt defeated Democrat Jason Kander by less than 3 points in 2016, which was a presidential year. One of his first Democratic challengers launched his campaign this year by mostly criticizing the other Missouri senator (Josh Hawley, who objected to the certification of Biden’s electoral win) and trying to tie Blunt to him.

The 10 seats below are ranked in order of most to least likely to flip. But it’s still early in the cycle, with candidate fields in flux, so be sure to check CNN’s next ranking to see how things have changed.

1. Pennsylvania

Incumbent: Republican Pat Toomey (retiring)

Pat Toomey 10 senate seats

Toomey’s announcement that he won’t seek reelection makes this state even more competitive than it otherwise would have been. Biden’s win here was a shift from four years ago, when Trump narrowly carried the Keystone State in 2016. That same year, Toomey won reelection by less than 2 points, defeating a Democratic nominee who attracted big-time endorsements but wasn’t the strongest campaigner. As with all open seats, the contours of this race will depend on who runs – and who wins each party’s nomination. On the Democratic side, Lt. Gov. John Fetterman, who finished third in the 2016 Senate primary, is the biggest name. The tattooed former mayor of Braddock, a working-class town east of Pittsburgh, sells himself as a progressive who can appeal to voters who saw something in Trump. Although his campaign is already touting big fundraising, he won’t have the field to himself. State Rep. Malcolm Kenyatta, a Biden surrogate during the 2020 campaign, has announced his bid and others – including members of the congressional delegation – could soon follow. Kenyatta was the first member of color who identifies as gay in the state Legislature. Without Toomey in the race, the Republican side is wide open and may attract a range of candidates on the spectrum of Trump loyalty, including prominent Trump critics like moderate former Reps. Charlie Dent and Ryan Costello.

2. Georgia

Incumbent: Democrat Raphael Warnock

Sen Raphael Warnock 10 senate seats

It’s time to talk about Georgia – again! Warnock won this seat earlier this year, defeating appointed Sen. Kelly Loeffler in a special election runoff to fill out the term of former Sen. Johnny Isakson. Warnock made history in January, becoming the first Black senator from the Peach State, and – along with Sen. Jon Ossoff, who won the state’s other Senate runoff – he flipped the chamber after Biden became the first Democrat in 28 years to carry the state. In 2022, Warnock is running for a full six-year term. There may not be as many resources poured into Georgia as there were when it was a presidential battleground or when it held simultaneous Senate runoffs, but the underlying factors that made the state competitive in 2020 aren’t going away. The Atlanta suburbs are still changing, attracting diverse and well-educated voters who lean Democratic. And all eyes are on Stacey Abrams to run for governor again, which would energize the Black voters Democrats need to turn out as part of their winning coalition. On the Republican side, former Sen. David Perdue now says he’s not running, but Loeffler – who recently launched a PAC aimed at conservative voter registration – is considering it, as is former Rep. Doug Collins, whose candidacy pushed Loeffler to the right last year when he failed to make it to the runoff. They all are coming off of losses, but Republicans feel better about their bench here than in some other offensive opportunities. As 2020 showed, Georgia is an evolving state, but it may not be as friendly to Democrats when it’s not a presidential year, and Republicans think they still have some sway with suburban voters if Trump isn’t on the ticket. It’s yet to be seen what role the former President could play in this race, but Republicans are hoping he’ll lay off attacking the state’s election system, which may have depressed GOP turnout in the runoffs.

3. Wisconsin

Incumbent: Republican Ron Johnson

Sen Ron Johnson 10 senate seats

Johnson hasn’t said whether he’s running for reelection, and it’s not clear which decision would give Republicans better odds of retaining this seat. If he retires, this race could get more competitive for Democrats as an open-seat contest, although Republicans feel confident they have a strong bench here. And if Johnson stays, he’s vulnerable, having ended 2020 with just over half a million dollars in the bank and barreling through 2021 with a penchant for saying conspiratorial things about the 2020 election and its aftermath (including questioning whether the attack on the US Capitol was an armed insurrection). The two-term senator seems to see some political benefit to catering to the former President, but that may not be the most viable path to reelection in a state that Biden reclaimed from Trump last fall. In 2016, Johnson dashed former Democratic Sen. Russ Feingold’s hopes of a comeback, beating him by about 3 points in a race that many observers had prematurely written off as a Democratic pickup. Whether Johnson stays or goes, Democrats have a decent chance to pick up the seat, although their field is still emerging. Milwaukee Bucks executive Alex Lasry is running, as is Outagamie County Executive Tom Nelson, a former lieutenant governor nominee who lost a House bid in 2016.

4. North Carolina

Incumbent: Republican Richard Burr (retiring)

Sen Richard Burr 10 senate seats

Trump’s win here last fall, coupled with the reelection of Sen. Thom Tillis, proved that there’s still a lot of red in this purple state. But with Burr not running for reelection, Democrats have a stronger chance of flipping his seat. Again, a lot depends on who the candidates are, but likely GOP candidates are already tripping over each other trying to criticize Burr’s surprise vote to convict Trump in his impeachment trial. Former Rep. Mark Walker, who declined to run for reelection in 2020 after his House district was redrawn, announced last fall that he’s running for the GOP nomination, but plenty of others could jump in, including former Gov. Pat McCrory, state GOP party Chair Michael Whatley, current members of the congressional delegation and even the former President’s daughter-in-law, Lara Trump. A Trump-style candidate could turn out his base but could also turn off voters in the growing suburban areas that have made this state competitive and allowed Democrats to win other statewide races. Democratic state Sen. Jeff Jackson is running, as is state Sen. Erica Smith, whom Republicans tried to prop up in the 2020 Senate primary because they believed she’d be a weak general election candidate. The Democratic field is likely to grow much larger.

5. Arizona

Incumbent: Democrat Mark Kelly

Mark Kelly AZ 1103

Kelly won in November, defeating Republican Martha McSally to fill the remainder of the late Sen. John McCain’s term. And like Warnock in Georgia, Kelly now has to run for a six-year term in a state that Biden flipped blue. This race is lower on the list of seats most likely to flip than Georgia, though, because it’s harder to see Republicans mounting an aggressive challenge to Kelly, who they admit ran a strong campaign and is an impressive fundraiser. McSally lost two successive Senate races, unable to perfect the delicate balance of appealing to Trump’s base without alienating suburban women, and there isn’t an obvious bench of viable candidates waiting in the wings. Arizona Republicans are deeply fractured, with those who have spoken out against Trump censured by the state party, which is now run by Trump loyalist and failed Senate candidate Kelli Ward. Those censured include GOP Gov. Doug Ducey, who has said he is not running for Senate. There’s plenty of time for him to change his mind before the filing deadline, though, and if he does, this race could look more competitive for Republicans.

6. Nevada

Incumbent: Democrat Catherine Cortez Masto

Sen Catherine Cortez Masto 10 senate seats

Cortez Masto was elected to her first term in 2016, beating Republican Joe Heck by about 2 points and becoming the first Latina senator. She went on to chair the Democratic Senatorial Campaign Committee, which should arm her with the contacts and fundraising ability to run a strong defensive campaign. Although Democrats have done well here in recent years, with Democratic Sen. Jacky Rosen unseating Republican Dean Heller in 2018, the Silver State is still competitive territory that Democrats shouldn’t take for granted. Biden won the state only by a little more than 2 points.

7. New Hampshire

Incumbent: Democrat Maggie Hassan

Sen Maggie Hassan 10 senate seats

Hassan, a former two-term governor, won her seat by the narrowest margins in 2016, defeating Republican Sen. Kelly Ayotte while Hillary Clinton also carried the state by less than a point. Fast-forward four years, and Biden carried the state by 7 points, while Democratic Sen. Jeanne Shaheen won a third term by nearly 16 points. All that makes New Hampshire look like a state that’s trending more blue. But either Ayotte or Gov. Chris Sununu could make this race competitive if they decide to run – and if and when that happens, this will move up the list of flippable seats. Sununu has said he’ll “take a look” at the race but wants to get through the legislative session first (which ends in June). His ambivalence could keep other Republicans out of the race until he decides, which could ultimately be a good thing for Hassan. But his potential candidacy is a threat Democrats are taking seriously.

8. Ohio

Incumbent: Republican Rob Portman (retiring)

Sen Rob Portman 10 senate seats

Trump’s strength in Ohio (he carried the state by 8 points in 2016 and 2020) is a clear sign of its partisan lean. But with Portman announcing in January that he won’t seek a third term, Democrats have a better shot at flipping the seat. (Portman last won reelection by more than 20 points, defeating former Gov. Ted Strickland, who was tarred as a DC liberal.) On the Democratic side, some of the bigger names to watch include Rep. Tim Ryan – who ran a short-lived 2020 presidential campaign and has long toyed with running statewide – and Amy Acton, the former state health director. But it’s the GOP primary that’s bringing the most drama so far, with the biggest names openly competing for a Trump endorsement while trying to tie one another to former GOP Gov. John Kasich, a noted Trump critic who endorsed Biden. Former state Treasurer Josh Mandel has adopted Trump’s rhetoric about a stolen election and attacked Rep. Anthony Gonzalez, a member of the delegation who voted for impeachment. Mandel has twice tried to knock out the state’s other senator, Democrat Sherrod Brown, but he lost in 2012 and dropped out in 2018, although he ended up with nearly $4 million left over for this race. Meanwhile, former state party Chair Jane Timken, who’s part of a wealthy steel manufacturing family, launched her campaign with a video filled with photos of Trump and her. “I cleaned house of the Kasich establishment,” she says. She’s now piled on Gonzalez, calling this week for him to resign. She’d be the first female senator from Ohio, and some Republicans like the idea of a Harvard-educated woman with Trump credibility campaigning in the suburbs.

9. Florida

Incumbent: Republican Marco Rubio

Sen Marco Rubio 10 senate seats

As in Ohio, Trump’s win here affirmed a lot of people’s thinking that Florida is a tough state for Democrats, although Republicans’ margins here are smaller. But unlike in the Buckeye State, the GOP incumbent is sticking around. Rubio last won reelection in 2016 (after dropping out of the presidential primary) by about 8 points, and since then, GOP Sen. Rick Scott – the chairman of the National Republican Senatorial Committee this cycle – unseated Democratic Sen. Bill Nelson. Any initial threat to Rubio from the Trump flank seems to have been dispelled with Ivanka Trump, the former President’s daughter, saying she won’t challenge him, but other challengers could emerge. Democrats to watch include Reps. Val Demings and Stephanie Murphy, both of whom have compelling personal stories and could help turn out diverse voters. Murphy, the co-chair of the moderate Blue Dog Coalition, launched a listening tour on issues last week – which is usually code for testing the waters of a campaign – but she’s said she may also hold out for a 2024 Senate run.

10. Colorado

Incumbent: Democrat Michael Bennet

Sen Michael Bennett 10 senate seats

After Biden’s nearly 14-point victory here last fall and Democrat John Hickenlooper’s defeat of GOP Sen. Cory Gardner, Colorado looks like a blue state. Gardner was one of the most vulnerable incumbents for all of the 2020 cycle, struggling to appeal to moderates and please the Trump base simultaneously. In an overt attempt to look bipartisan, one of his ads even flashed a picture of Bennet, who had endorsed his opponent. Now it’s Bennet’s turn to run for reelection. And despite those recent victories, Democrats aren’t taking this race for granted in a state that still has plenty of conservative areas. Bennet is known as a cerebral legislator, but he earned national attention for a fiery Senate floor speech lambasting Texas’ Sen. Ted Cruz that went viral in 2019. He briefly ran for the 2020 Democratic presidential nomination, but even though some of his more moderate votes distinguished him from other contenders he failed to stick out or gain traction. He was first appointed to this seat in 2009, then won a full term in 2010 by defeating Ken Buck, now a Republican congressman who has said he’s not running for Senate this year. The Democrat didn’t face much of a challenge in 2016, with observers writing off his race well before it was over. But in the end he won reelection only by about 6 points. And although Colorado has trended more blue in the last six years, Bennet’s relatively close 2016 margin is a reason not to move this race off the list quite yet.

CORRECTION: An earlier version of this story misstated Pat Toomey’s title in one instance. He is a senator from Pennsylvania.