BOZEMAN, Mont. — Voters caught up in this state’s exceedingly close Senate race could be forgiven for believing Gov. Steve Bullock, the Democratic candidate, was sharing the ballot with Senator Chuck Schumer of New York, the minority leader.
An incessant stream of Republican-funded advertisements depicts Mr. Bullock, a popular two-term governor, alongside Mr. Schumer and other leading Democrats such as Speaker Nancy Pelosi. Senator Steve Daines, the Republican incumbent battling for political survival in the conservative-leaning state, regularly tries to tie his opponent to the national party, asserting that Mr. Bullock is Mr. Schumer’s “lap dog.”
“Steve Bullock in the Senate puts Chuck Schumer in charge,” the latest ad from the National Republican Senatorial Committee said. “Think about that.”
Mr. Bullock doesn’t think much about it.
“I say it is all B.S.,” he said in an interview as snow descended upon Montana and shut down campaign appearances already curtailed by the coronavirus, which has hung over the election season and was surging anew in the Mountain West. “Montanans see through some of the stuff where they are trying to turn me into something they don’t recognize.”
But while the governor resented the ad’s implication, in one respect it was absolutely true: If Mr. Bullock is elected to the Senate, Mr. Schumer will almost certainly be in charge as the majority leader.
Here in Montana and in crucial battleground states across the country, Republicans are playing defense in a struggle for control of the Senate. Dragged by President Trump’s struggles even in conservative states and confronting a phalanx of Democratic opponents who have raked in extraordinary sums of cash to challenge them, Republicans privately acknowledge that their majority is hanging by a thread.
The Montana race, described by strategists for both parties as a coin flip, is one of a handful of contests that will determine control of the Senate and the ability of the next president to pursue his agenda, fill a cabinet and win the judicial confirmations suddenly at the forefront of the nation’s political dialogue. Democrats have other narrow paths to Senate power, but a Bullock victory would essentially assure it and signal Democratic gains elsewhere.
As the race entered its final days, Republicans led by Mitch McConnell, the majority leader, were being forced to shore up candidates in traditionally safe locations such as Alaska, Kansas and South Carolina. Democrats believe they are already on track to win Arizona and Colorado and are on the hunt for a half-dozen others starting with Iowa, Maine and North Carolina. Georgia is suddenly looming as a real opportunity for Democrats, with both of the state’s Republican senators at risk.
Democrats conceded they were likely to lose in Alabama, where Senator Doug Jones is running for re-election after an upset victory in a special election in 2017, and are keeping a close watch on Michigan, where Senator Gary Peters is facing John James, a Republican who was receiving a last-minute surge of outside money. But they were eyeing significant pickups elsewhere.
With Republicans holding the Senate majority by a margin of 53 to 47, a net gain of three seats would put Democrats in control should former Vice President Joseph R. Biden Jr. win the presidency; four would be required if Mr. Trump is re-elected.
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Mr. Schumer called the battle for the Senate a “nail-biter,” but said the outlook for Democrats was far brighter than it was only a few months ago.
“If you would have told me in early January that we would have a good chance of winning the Senate, I would have said that is a real long shot,” he said in an interview. “Now we are in the ballpark because of the strategic decision to expand the seats that are in play.”
Republicans quietly agree that their prospects have dimmed considerably. One Senate Republican leader said privately that the party could end up with anywhere from 47 to 52 seats.
“It is a 50-50 proposition,” said Mr. McConnell, Republican of Kentucky, who appears to be safely headed toward re-election himself, about his chances for remaining majority leader. “We have a lot of exposure. It is a dog fight out there.”
Republicans are battling the brawl from a weakened position, given Mr. Trump’s woes and the record-crushing levels of money Democrats have raised for Senate races. Democratic challengers have flipped the usual formula on its head and outperformed Republican incumbents when it comes to contributions, providing ample resources for TV advertising even as the traditional campaign model has been upended by the pandemic.
Here in Montana, at least an estimated $120 million will be spent, while in Maine, more than $100 million is being poured into the showdown between Senator Susan Collins, a Republican seeking a fifth term, and Sara Gideon, the Democratic speaker of the State House. The race in North Carolina, where Senator Thom Tillis, a Republican, is trying to hold on against Cal Cunningham, a Democrat, is set to be the most expensive Senate contest ever, with well over $200 million expended.
While Democratic candidates are still outspending Republicans in most of the marquee races during the final weeks, the Senate Leadership Fund, a super PAC affiliated with Mr. McConnell, has dumped more than $82 million into the effort since early October, helping close a money gap Republicans attribute to the success of online small-donor Democratic fund-raising.
“We are being outspent dramatically, like so many other races around the country,” said Senator John Cornyn, a three-term Texas Republican who finds himself in an unexpectedly tough match against M.J. Hegar, a Democratic military veteran. “We have been a little late to the dance and I think we are going to have to learn from the way Democrats raise money. It is no longer like going to a dinner and a cocktail party.”
The fact that Mr. Cornyn finds himself sweating is testament to how Democrats have opened multiple potential routes to a Senate majority while Republicans have failed to put their usual strongholds out of reach. Democrats have been unable to put away Ms. Collins, whom they saw as exceptionally vulnerable after her vote in 2018 to confirm Judge Brett M. Kavanaugh to the Supreme Court. But Republicans now worry the voting system in Maine — in which voters’ second-choice candidate is counted if no candidate earns 50 percent — could cost her on Tuesday.
Democrats have focused relentlessly on health care in their campaigns, scorching Republicans for their yearslong drive to overturn the Affordable Care Act and its protections for pre-existing conditions. The power of the message has been amplified by the pandemic and public anxiety about health costs. Republicans have pushed back with pledges to guarantee coverage but have not produced specifics on how they would do so.
The challenge for Democrats is that to win the Senate, they must prevail in states Mr. Trump carried in 2016 and is likely to carry again this year such as Iowa and Montana, though Biden victories in places like Arizona and North Carolina would make their task easier. Republicans had hoped recent revelations that Mr. Cunningham had an extramarital affair would help rescue Mr. Tillis in North Carolina, but polls suggest it has not happened and Mr. Tillis remains in trouble.
As Mr. Biden shows surprising momentum in Georgia, where two Republican Senate seats are up for grabs, Republicans have become increasingly concerned about the outlook for Senator David Perdue, who drew widespread criticism for appearing to mock the name of Senator Kamala Harris, his colleague and the Democratic vice-presidential candidate. Polls show the Democratic challenger Jon Ossoff near the 50 percent threshold needed to avoid a runoff on Jan. 5, though the race for Mr. Perdue’s seat could spill into next year along with the fight for the one occupied by Senator Kelly Loeffler, a Republican who was appointed to the Senate last year. Such runoffs make it conceivable that control of the Senate will not be decided until January.
With Mr. Trump in real danger of losing, Senate Republicans are raising the alarm in more conservative states that a Republican Senate is needed to maintain a check on Democrats should Mr. Biden win the presidency and Democrats hold the House as expected. Republicans are pointing to Democratic support for calls to eliminate the filibuster and add seats to the Supreme Court as evidence of what Democrats plan.
“What happens if they have all the power?” a new National Republican Senatorial Committee ad airing against the Democratic candidate for Senate in Iowa, Theresa Greenfield, asks as ominous background music plays. “They will change the Senate rules to advance their liberal agenda.”
In Montana, a state awash in campaign advertising and negative political attacks, Republicans have sought to portray Mr. Bullock, whose job performance ratings have remained strong as he has managed the state response to the pandemic, as a politician who turned to the left during a failed presidential primary run.
“Steve Bullock is too liberal for Montana,” Mr. Daines said in a brief interview.
But Mr. Bullock proved in 2016 that he could draw the support of conservatives, narrowly winning re-election even as Mr. Trump easily carried the state. As a result, Republicans are trying to persuade voters that sending him to Washington would be more dangerous than putting him in charge of their state.
“They might like him as governor, but ultimately he is playing for the wrong team and would green-light Democrats’ extreme agenda in the Senate,” said Jesse Hunt, a spokesman for Republicans’ Senate campaign organization.
Mr. Bullock scoffs at the characterizations. His campaign advisers believe that a final campaign push focused on digital advertising targeted at the few remaining undecided voters and an intense outreach operation can bring a victory in a contest that both sides believe will be settled by a few thousand votes at most.
“The same person that Montanans elected twice governor and is now running for the Senate is going to be the same person when I show up in D.C.,” he said. “I’m not going to drink the water.”
Nicholas Fandos contributed reporting from Washington.