In one of the nation’s top Senate races, the most important candidate might be one who hasn’t actually entered the contest.
The scramble to challenge Senator Jon Tester, Democrat of Montana — already a heated battle with profound national implications and the acute intensity of a family drama — has increasingly focused on Representative Matt Rosendale, an anti-abortion, election-denying Republican agitator.
Both parties are using him as a pawn in their electoral chess match: Establishment Republicans, who have aligned behind Tim Sheehy, a wealthy businessman, are trying to keep Mr. Rosendale out of the race, while Democrats appear to be helping clear a path for his arrival.
Mr. Rosendale’s entry appears imminent. On Thursday, he said on a podcast that he had told Senator Steve Daines, a fellow Montanan who oversees the Republican Senate campaign arm, that he was going to run for Mr. Tester’s seat.
Such a move would complicate plans from Mr. Daines, who has been trying to avoid divisive primaries and the elevation of polarizing far-right candidates. Often endorsed by former President Donald J. Trump, such candidates have won Republican primaries in recent cycles only to fall short in general elections decided by moderate swing voters.
This year, Republicans have an advantageous map as they look to retake control of the Senate, where the races seen as most competitive are in two states — Montana and Ohio — that Mr. Trump won easily in 2016 and 2020. But the Democrats who hold those seats, Mr. Tester and Senator Sherrod Brown of Ohio, are both popular and have experience navigating rugged political terrain.
Mr. Tester has already beaten Mr. Rosendale once, in 2018. Since then, Mr. Rosendale voted to overturn the 2020 election and, last year, helped set off chaos in the House when he and a small group of Republicans banded together to overthrow their party’s speaker, Kevin McCarthy. He has also voiced support for banning abortion without exceptions.
The historical difficulty of defeating an incumbent like Mr. Tester, even in a deep-red state, only adds to the urgency for Mr. Daines to avoid a contentious primary that would force Republicans to fire political attacks at each other instead of the rival party.
It hasn’t been going well.
A super PAC that has supported Mr. Daines in Montana, known as More Jobs, Less Government, has been preparing for Mr. Rosendale’s entry into the race by testing a series of attack ads, previously unreported, that are aimed at sowing doubts about his conservative credentials.
As part of the testing, the super PAC sent Montanans a lengthy online survey that asks questions about issues and political candidates, and that also seeks information about the respondents.
One question, earlier reported by The 19th, asks respondents their gender and gives three eyebrow-raising options: male; female, homemaker; and female, working woman.
The group has already spent nearly $1 million on pro-Sheehy spots in Montana, according to AdImpact, a media-tracking firm. In a previously unreported poll for the super PAC, conducted by Tony Fabrizio, Mr. Trump’s longtime pollster, Mr. Sheehy was leading Mr. Rosendale in a hypothetical primary, 48 percent to 24 percent.
In an interview on Thursday with Stephen K. Bannon’s “War Room” podcast, Mr. Rosendale said that Mr. Daines had repeatedly pressured him to stay out of race. He recounted that in one instance, an ally of Mr. Daines — whom Mr. Rosendale described as a “political soldier” — asked a Rosendale confidant, “What is it going to take to keep you out of this race?”
Mr. Rosendale said he had received a direct warning from Mr. Daines in November 2022 to “tone it down” and stop “making problems” in the House.
Mr. Rosendale said that Mr. Daines had told him that billionaires “are going to spend a lot of money against you,” adding, “What do you want me to tell them?”
In Mr. Rosendale’s retelling of the conversation, he responded that he would win the primary, and such donors had “better save their money for the general election.”
Asked to respond to Mr. Rosendale’s accusations, Mike Berg, the communications director for the National Republican Senatorial Committee, said, “It is unfortunate that Congressman Rosendale is casting blame on others for his fund-raising challenges and trouble keeping staff.”
This week, the N.R.S.C. ordered television advertising worth six figures aimed at promoting Mr. Sheehy.
Last summer, more than three dozen Republicans in the Montana Legislature — including Jason Ellsworth, the Senate president, and Matt Regier, the House speaker — signed a letter urging Mr. Rosendale to run.
And in November, a Republican former state representative, Roger Koopman, accused Mr. Sheehy and his allies of trying to impose party unity with “a variety of intimidating messages and veiled threats of retaliation.”
Montana Democrats, meanwhile, have quietly been running a series of online ads that appear to promote Mr. Rosendale in the expected Republican primary by calling attention to his conservative credentials — though the state Democratic Party disputes that the Facebook ads are intended to help him.
The social media spots are subtle and cheap, but resemble a controversial strategy Democrats used in the 2022 cycle to help right-wing candidates who they believed would be easier to defeat in a general election. Those bets worked in places like Michigan, where Democrats flipped a House seat by defeating John Gibbs, a Trump-backed candidate, and Pennsylvania, where Doug Mastriano’s fringe campaign for governor lost by double digits.
Montana Democrats have spent about $27,000 promoting the spots on Facebook since last month, according to disclosures maintained by Facebook. The advertisements are produced by a group, Treasure State Truths, that has links to a Democratic consulting firm.
Some of the Democratic Facebook ads are focused on Mr. Rosendale, while others are focused on Mr. Sheehy.
The Sheehy-specific ads cast direct judgment on the candidate or urge viewers to take action against him. “We just can’t trust what Tim Sheehy says,” several spots say. “Tell Tim Sheehy and out-of-state millionaires to stop hurting Montana,” another says.
But the ads focused on Mr. Rosendale are more hands-off.
One spot, for example, amplifies his support for banning abortion without exceptions by pointing to an opinion column that praises the Montana Republican for his support.
The only line of text in the ad quotes from the column, saying that Mr. Rosendale “has always been an outspoken and unapologetic advocate” of abortion restrictions.
All of the spots are marked with a disclosure that identifies the Montana Democratic Party as the sponsor. Democrats said the Facebook ads were in line with the messaging the party has used against both Republicans.
On television, a super PAC that appears to have ties to Democrats has spent more than $5 million running ads that attack Mr. Sheehy. The group, known as Last Best Place, has not disclosed its donors. Politico reported in September that the group’s airtime had been bought by a company called Mountain Media Agency, which shares an address with Old Town Media, a Democratic campaign firm.
“Democrats are clearly reprising their cynical strategy of meddling in Republican primaries,” Mr. Berg said.
Hannah Rehm, a spokeswoman for the Montana Democratic Party, said the Facebook spots were not meant to promote Mr. Rosendale. Republicans, she said, were overreacting to the ads because of the fight between Sheehy allies and Rosendale supporters.
“Might be that the call is coming from inside the house,” she said.