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

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

Republicans need to win just two seats to gain a Senate majority and are widely believed to have picked up a seat in West Virginia.

Sheehy, 38, is a former US Navy SEAL and a wealthy businessman. He has 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 victory in 2006, also against a three-term incumbent.

Tester has tried to turn the race into 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 the Roe vs. Wade.

He labels Sheehy an unwanted outsider who is “part of the problem” of rising taxes after home prices have risen in many parts of the state amid a housing shortage.

Sheehy has said his flight was prompted by the disastrous US military withdrawal from Afghanistan. The political rookie’s campaign has occasionally failed: He admitted to lying about the origin of a gunshot wound to his arm and faced backlash for derogatory comments he made to his supporters about Native Americans that were obtained through a tribal newspaper.

Still, Republicans remain confident that they have finally gotten Tester into office, 18 years after he entered the Senate. Recent polls show Sheehy making gains in a state that Trump won by 17 percentage points in 2020.

The state has drifted further to the right with each successive election cycle, driven in part by newcomers like Sheehy, who came to Montana in 2014 to start an aerial firefighting company.

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

In an effort to blunt the attacks, Tester skipped the Democratic National Convention last month, refusing to endorse Harris and avoiding mention of her during the campaign. He pushed back against the government over tougher pollution rules on coal-fired power stations and urged it to do more on immigration.

Sheehy has no political record to criticize, but Tester and Democrats have pointed to his past comments in support of abortion restrictions. They claim Sheehy would help ban abortion in Montana.