After 18 years working to topple Montana Democratic U.S. Sen. Jon Tester, Republicans in Big Sky Country see potential victory and control of the Senate majority within their grasp in an increasingly acrimonious contest that's shattering campaign spending records.

Montana voters, meanwhile, are getting worn out — deluged by negative ads on their TVs, radios, phones and in their mailboxes.

Tester won by a narrow, 3,500-vote margin in 2006 and has held on for three terms despite a dramatic political re-alignment across the U.S. Northern Plains. He's confronting what analysts say is his most serious challenge yet in Republican Tim Sheehy, a former U.S. Navy SEAL and wealthy aerospace executive aligned with former President Donald Trump.

At a weekend Tester rally in Bozeman, Montana — where Sheehy in August held an event with Trump that drew thousands of people — the crowd for the incumbent lawmaker numbered in the dozens.

Josh Olsen, an outdoor guide and Tester voter, worried that as Montana's population expands, its electorate is becoming too partisan to back the grain farmer from the tiny town of Big Sandy who is counting on his cross-party appeal to give him another term.

"A hundred percent I'm worried about it," said Olsen. "There's more partisan people coming here...If they're coming here and they're Republicans, they're voting for Sheehy."

Tester, 68, entered office as one of a half dozen Democratic senators in a five-state region stretching from Nebraska to Canada. He's the last one still in office, and Republicans have spent years trying to chip away at his support, particularly in rural areas.

Montana is one of the least-densely populated states in the U.S., and only about a quarter of its residents live in cities of 50,000 or more.

"Outside the cities of Montana, Republicans have made gains in most of the towns and rural counties," said political analyst Jeremy Johnson at Carroll College. "That's a challenge for Democrats."

Republicans have a two-seat deficit in the Senate.

Democrats, looking to retain their majority, are on track to outspend Republicans by almost $50 million in the Montana race. Total spending is expected to exceed $315 million, or about $487 for each of the state's 648,000 active registered voters — a record for a congressional race on a per-voter basis, according to party officials.

The cash fuels dueling ad campaigns with similar goals: Tear down the opponent.

A Sheehy campaign ad talks about rampant corruption in Washington and calls Tester "one of the worst offenders." A Tester ad labels Sheehy a "fake cowboy" and attacks him for lying about a bullet wound in his arm.

Former Montana Gov. Marc Racicot called the flood of money into the sparsely populated state "absolutely obscene." It comes more than a decade after the U.S. Supreme Court struck down political spending restrictions on corporations and unions.

"You can't stand to even turn on TV," Racicot said in an interview. "You're just confronted constantly with this anger, grievance, sloganeering — everything that goes on in these campaigns — because there's so much money involved. It's an abomination."

If Sheehy wins, a Republican-majority Senate can hamstring Democrats' agenda should Vice President Kamala Harris win the White House. Similarly, a Tester victory could help Democrats counter a Trump administration's actions.

Much of the money traces back to shadowy political committees with wealthy donors.

The non-partisan Campaign Legal Center has sued over alleged financial transparency violations by a pro-Tester group, Last Best Place PAC, that has amplified some of the most incendiary claims against Sheehy. Another complaint from the advocacy group charges that a straw donor was used to conceal more than $2.5 million in contributions to political committees, including one supporting Sheehy.

The allegations are unlikely to be resolved before the election.

Trump won Montana overwhelmingly in 2016 and 2020. The 2024 election is the first where both Tester and Trump are on the ballot.

Sheehy's campaign themes largely mirror those of Trump and the national party. He rails against immigration, inflation and social issues such as transgender children in sports.

Democrats hitched Tester's campaign to abortion and women's health care, hoping the backlash that surfaced even in red-state elections after the Supreme Court's 2022 decision to overturn Roe v. Wade will continue.

Tester has distanced himself from Harris and other Democratic leaders. That political isolation reflects the GOP's gains over the past two decades with rural voters who once backed Democrats.

Tester's campaign has outraised Sheehy's by more than three-to-one, propelled by a massive influx of small out-of-state donations after Democrats raised alarms over the race.

"Jon Tester has more money but dollars don't vote," Sheehy's campaign said in a statement. "Montanans just want common sense: a secure border, safe streets, cheap gas, cops are good and criminals are bad, boys are boys and girls are girls."

Sheehy, who arrived in Montana a decade ago and compares himself to early European settlers, has sought to turn Tester's Senate tenure into a liability.

Republicans allege a pattern of campaign donations flowing to the Democrat from industries that needed his vote. Tester previously faced scrutiny over donations from bank executives affected by a 2018 regulatory roll back and Lockheed Martin employees who benefited from a 2021 defense bill. There's no indication of wrongdoing or that the contributions swayed Tester.

He ranks as the number one recipient of lobbyist cash among members of Congress, with $500,000 in donations this election cycle, and has raised $88 million overall.

Tester said in an interview following the Bozeman rally that he doesn't know who donates to him or if any lobbyists were there that night.

"I've got policies to write, people to get on board," he said. "If it makes sense for Montana, I'll support it."

Sheehy has money pouring in from national groups, too. He's received $109,000 from lobbyists and previously lobbied government officials himself, seeking business for the aerial firefighting company he founded with his brother.

Sheehy has no prior political experience and has avoided in-depth interviews. He's faced blowback over derogatory remarks he made to supporters about Native Americans and questions about the bullet wound in his arm, which he has said came from a firefight in Afghanistan.

Sheehy told a Glacier National Park ranger in 2015 that the wound was self-inflicted. Ranger Kim Peach, now retired, last week publicly accused Sheehy of lying when he says it was from combat.

Peach subsequently appeared in an advertisement from a pro-Tester PAC that he acknowledged was recorded weeks ago, despite telling AP last week that he was unconnected to Democratic groups.

Republicans said Peach wasn't credible.

"He's shifting his story left and right because he is a liar and a Democrat partisan," said Mike Berg with the National Republican Senatorial Committee.

Tester said Sheehy should release his medical records to clear up the dispute, adding: "Stolen valor is a huge problem."