BILLINGS, Mont. (AP) — U.S. Sen. Jon Tester of Montana is fighting to hold on to his seat and prevent a Republican takeover of the Senate as the three-term lawmaker faces GOP challenger Tim Sheehy in a Monday night debate.

Tester is the last remaining Democrat to hold high office in Montana and the race is on track to be the most expensive in state history. Republicans party leaders including former President Donald Trump handpicked Sheehy in hopes of toppling Tester, a 68-year-old farmer.

Republicans need to pick up just two seats to take the Senate majority and are widely considered to have a lock on one, in West Virginia.

Sheehy, 38, is a former U.S. Navy SEAL and a wealthy businessman. He’s sought to erode Tester’s longstanding support among moderates by highlighting the lawmaker’s ties to lobbyists. That’s a tactic Tester himself used successfully in his first Senate win in 2006, also against a three-term incumbent.

Tester has attempted to make the race a referendum on reproductive rights for women, closely tying his campaign to a November ballot initiative that would enshrine abortion rights in Montana’s constitution following the overturning of Roe vs. Wade.

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He’s labelled Sheehy as an unwelcome outsider who is “part of the problem” of rising taxes after home values increased in many areas of the state amid a housing shortage.

Sheehy has said his run was motivated by the disastrous U.S. military withdrawal from Afghanistan. The political rookie’s campaign has stumbled at times: He admitted to lying about the origin of a bullet wound in his arm and has suffered backlash for derogatory comments he made to supporters about Native Americans that were obtained by a tribal newspaper.

Yet Republicans remain confident they’ve finally got Tester on the ropes 18 years after he entered the Senate. Recent polls suggest Sheehy making gains in a state that Trump won by 17 percentage points in 2020.

The state has drifted farther right with each subsequent election cycle, driven in part by new arrivals such as Sheehy, who came to Montana in 2014 to start an aerial firefighting business.

Sheehy has embraced his status as an outsider and said he would speak for both newcomers and longtime residents. He repeatedly tries to lump Tester with President Joe Biden and Vice President Kamala Harris, highlighting public dissatisfaction over the administration’s struggles to stem illegal immigration on the southern border.

Seeking to blunt the attacks, Tester skipped the Democratic National Convention last month, declined to endorse Harris and avoids mention of her on the campaign trail. He’s opposed the administration over tighter pollution rules for coal plants and pressed it to do more on immigration.

Sheehy has no political track record to criticize, but Tester and Democrats have pointed to his past comments supporting abortion restrictions. They claim Sheehy would help “outlaw abortion” in Montana.