Washington (CNN) -- It's the biggest storyline heading into next year's midterm elections: Will Republicans succeed in taking control of the U.S. Senate?
Republicans argue that an announcement over the weekend vastly improves their chances. Democrats vehemently disagree.
The news from Montana's former Democratic Gov. Brian Schweitzer. The man known for his signature bolo tie said he would not run for Senate next year, as many Democrats had expected.
Schweitzer captured nearly two-thirds of the vote in his 2008 re-election as governor of the red state and was seen by many as the best chance for the party to keep the seat of retiring Sen. Max Baucus blue.
Schweitzer has recently faced potentially damaging stories about his ties to a political nonprofit group and its disclosure practices.
But sources said that may have just been the tip of the iceberg when it came to opposition research Schweitzer may have faced had he decided to launch a Senate bid.
But Schweitzer denied that impacted his decision.
"This isn't my first rodeo," he told The Associated Press.
A Republican source said Schweitzer's decision is a "sea change in the 2014 atmosphere."
"Winning the majority is something we've always thought not only possible, but our mission and bar for success, yet many others remained skeptical -- particularly donors and the pundit class," said the source, who asked to remain anonymous to speak more freely. "This development starts to break through that wall and crumple the belief that a Republican majority is out of reach."
Next year, the Democrats will try to maintain their majority in the Senate, where they currently hold a 54-46 edge (including two Independents who caucus with the party) over the GOP. They hope to expand that to 55-45 following October's special Senate election in New Jersey, which they are favored to win.
But they most likely will be defending 21 of 35 seats up for grabs in November 2014.
Besides Montana, Republicans are also optimistic about capturing seats in two other states that vote red in presidential elections: neighboring South Dakota, where Democratic Sen. Tim Johnson is not running for re-election; and West Virginia, where Democratic Sen. Jay Rockefeller is retiring.
If the GOP captures Montana, South Dakota, and West Virginia, it only needs three more seats to reach the magic number of 51.
Republican eyes are focused on four Democrats facing tough battles next year: Mark Pryor of Arkansas, Mary Landrieu of Louisiana, Kay Hagan of North Carolina, and Mark Begich of Alaska.
Take three of those four races and the Republicans will do what they couldn't do in 2010 and 2012: Win back the majority.
"We are in a strong position not only to win in Montana, but national pundits are now beginning to recognize something that we've been saying since February: Republicans are positioned to win the Senate majority in 2014," National Republican Senatorial Committee spokesman Brad Dayspring said in an e-mail on Monday.
Democratic Senatorial Campaign Committee deputy executive director Matt Cantor disagrees.
"Republicans have been feeling good about their chances in the U.S. Senate for six years and they're still in the minority," Cantor told CNN. "The math still strongly favors Democrats and so does history. Democratic incumbent senators are very tough to beat. Only three Democratic incumbents have lost re-election in a decade."
Sources say that recent stories about Schweitzer were just the tip of the iceberg when it came to opposition research he might have faced if he had launched a Senate bid.
"Democrats and Republicans agree on Schweitzer: There was a treasure trove of opposition research. It's highly unlikely that his candidacy would have been successful," said a Democratic source who asked for anonymity in order to speak more candidly. "Now we have a list of prospects, and while there's no question that they begin as underdogs in a red state, Democrats are better off with a candidate that can win rather than a candidate that would self-combust before the 2014 elections."
But Schweitzer had statewide name recognition that no other Democrat may be able to match, said Nathan Gonzales, political editor of the non-partisan Rothenberg Political Report.
"Republicans have a narrow path to the Senate majority in 2014, but it got a little wider when Schweitzer decided not to run," Gonzales said. "We still don't know who the candidates will be, but Schweitzer was a proven vote-getter with nearly universal name identification. No Democrat will start from the same place he would have. Democrats can win in Republican-leaning states, but it takes someone with proven independence from the national party. We'll see if Democrats can find that candidate."
Democrats are also defending retirements in Iowa, where Sen. Tom Harkin is not running for a sixth term, and Michigan, where Sen. Carl Levin is not bidding for a seventh term. As of now, both of those seats appear to be safer for the Democrats.
The two Republican retirements are in states that appear to be safe for the GOP: Sen. Mike Johanns of Nebraska and Sen. Saxby Chambliss of Georgia, although Democrats think an ugly primary in Georgia could lead to a very conservative nominee who would be beatable in the general election.
The only other seat Democrats hope to put in play is in Kentucky, where Senate Minority Leader Mitch McConnell, who's running for a sixth-term, is facing a challenge from Democratic Secretary of State Alison Lundergan Grimes.