Republican Mehmet Oz, left, and Democrat John Fetterman are running for Senate.
CNN  — 

No Senate race in the country has received as much money and attention as the hotly contested and at-times divisive contest between Republican Mehmet Oz and Democrat John Fetterman in Pennsylvania.

And with over one million ballots already cast and Election Day just hours away, the reason is clear: Republican Sen. Pat Toomey’s retirement in a state President Joe Biden won two years ago has created Democrats’ best opportunity to pick up a seat and save their narrow majority. For Republicans, holding the seat is key to toppling that majority.

“This is a must-win race,” said Steven Law, president of the Senate Leadership Fund, the preeminent Republican Senate super PAC that has blanketed the state with tens of millions in ads attacking Fetterman. “We believe if we win Pennsylvania, we win the majority.”

With even upbeat Democrats conceding the party is unlikely to keep control of the House on Tuesday, Senate control is arguably the most closely watched battle on Election Day. Republicans only need a net gain of one seat. Democrats are focused on protecting incumbents in Nevada, Arizona, New Hampshire and Georgia and possibly flipping seats in Pennsylvania, Wisconsin, North Carolina and Ohio.

But it is the race in Pennsylvania that both parties view as critical. From Labor Day through Election Day, nearly $160 million will be spent on ads by both parties, more than any other Senate race, according to ad tracker AdImpact.

“The bottom line is if Democrats are able to flip a current Republican-held seat, there is likely no path for the Republicans to get to 51 votes in the Senate,” said Mike Mikus, a Democratic operative based in Western Pennsylvania. “It gives the Democrats some breathing room because if one of the incumbents goes down, this is the buffer. And the inverse is true. If for some reason we can’t win here, it is going to be a bad night in multiple states.”

That importance was clear as Oz and Fetterman crisscrossed the commonwealth in the final week of campaigning, trying to appeal to last-minute voters and urging people who have long decided who they’re voting for to now get their friends and family to the polls.

“It’s a jump ball,” Fetterman said bluntly on Sunday in Harrisburg. “On Tuesday, it’s going to come down to every single vote.”

“I have one job. … Win this race. You are the key,” Florida Sen. Rick Scott, the head of the Republican committee tasked with taking control of the Senate, said introducing Oz on Thursday. “You want a majority in the Senate? Yes. It comes right through Pennsylvania.”

Recent polling shows Fetterman with a slight edge, but with a narrower margin than he enjoyed over the summer. A Marist College poll conducted from late October to early November found Fetterman with 50% support among registered voters, compared to Oz’s 44%. A New York Times/Siena College survey among likely voters found a similar result: Fetterman with 49% support to Oz’s 44%.

Few races across the country have experienced the kinds of ebbs and flows that have altered the Pennsylvania contest, where, as of Sunday, 1,085,353 ballots had already been cast, according to Catalist, a company that provides data, analytics and other services to Democrats, academics and nonprofit issue-advocacy organizations.

While Fetterman easily won the Democratic primary in May, carrying every Pennsylvania county, Oz was forced to spend millions in a contentious battle that divided all segments of the Republican electorate, leaving the candidate battered and hamstrung by underwater favorability ratings as the general election began.

Just days before the primary, though, Fetterman suffered a near fatal stroke, forcing him off the campaign trail for two months, impacting his speech and ability to process what he is hearing and injecting a sizable unknown in a race that forced the Democrat to recover in public.

Despite the stroke, Fetterman and his campaign used the summer to open a sizable lead over Oz, seizing on his scant ties to Pennsylvania, the Republican questions about him that were opened up during the primary and the Democratic energy felt around the Supreme Court’s decision to overturn Roe v. Wade. But Oz’s campaign kicked into gear in the late summer, helped by millions in outside spending that subjected Fetterman to repeated attacks on crime. The surge in spending was followed by polls showing a tightening race.

Republicans had hoped, however, that the first and only debate between Oz and Fetterman in October would fully put Oz over the top, given some of the Democrat’s struggles during the debate highlighted his ongoing stroke recovery. But only 3% of voters in a recently released Monmouth University poll said the debate had caused them to reconsider their choice in the race.

For top Republicans, the mere fact that the race is this close after the slog of a summer Oz experienced is a triumph.

“Oz has done a very good job of answering those concerns by attending everything and being everywhere,” said Matthew Brouillette, the founder and president of Commonwealth Partners, a top Republican super PAC in Pennsylvania. “Once people get to hear and know him, it’s easy to see them move from skeptical to supportive. I myself had that experience.”

A top Republican operative working on Senate races was more direct about how worried the party was over the summer, as Oz looked unable to pivot from the primary to the general.

“It was clear, given the body of evidence, that the way the race was trending it was in danger of slipping off the map entirely,” this person said. “It would have been disastrous.”

The importance of the race was on full display Saturday, when the last three presidents – one current, two former – traveled to the state to urge the same voters that had helped them win the presidency to turn out and help their party.

“This could be the vote that’s going to make the difference between a country and not a country,” former President Donald Trump said in his push for Oz. “It could be 51 it could be 50,” he said of the balance of power in the Senate. If it’s “49 for the Republicans, this country – I don’t know if it’s going to live for another two years.”

Biden and former President Barack Obama also ratcheted up the pressure on the commonwealth.

“This ain’t your father’s Republican Party. This is a different breed of cat,” Biden said, urging voters to back Fetterman.

The former Democratic president offered a more dire warning, cautioning Democrats against making the same midterm mistakes that altered his presidency.

“I can tell you from experience that midterms matter, a lot,” said Obama, who lost his House majority in 2010 and Senate majority in 2014. “When I was president, I got my butt whopped in midterm elections. … I am not big on looking backwards, but sometimes I can’t help but imagine what it would have been like if enough people had turned out in those elections.”

Few voters paying attention to the race remain undecided, a reality of a polarized commonwealth that that has seen hundreds of millions spent on political campaigns this cycle.

“I decided a long time ago that I was voting for John Fetterman,” said Michelle Schofield, a 48-year-old mother from Delaware County. “This election is so much bigger than just John Fetterman. We need him in the Senate.”

For Oz, that dynamic is amplified because he, as a celebrity television doctor who spent years on national television, is almost universally known.

“I love that man. I think he will do a good job,” said Sarah Barrett, an independent voter from Moscow, Pennsylvania. “When I saw that he was running, I was just like, yes, because I feel like he cares about people. I felt that watching the show. I feel that now.”

This story has been updated with additional developments.

CNN’s Kit Maher contributed to this report.