BIGFORK, Mont. (AP) -- After 17 years in the U.S. Senate, Democrat Jon Tester is a well-known commodity in Montana -- a plain-spoken grain farmer with a flattop and a carefully cultivated reputation as a moderate.

The 67-year-old lawmaker smiled and laughed his way through the crowd at a Veterans Day event in Bigfork, a small town on Flathead Lake where the population has surged in recent years. He chatted with veterans who supported him and some who didn't, then stood behind a lectern in the Bigfork High School gymnasium to promote his biggest recent accomplishment: expanded federal health care for millions of veterans exposed to toxic smoke at military "burn pits."

Tester has survived three close elections and a changed national political landscape to emerge as the lone Democrat still holding high office in Montana. The 2024 election brings possibly his stiffest challenge yet: Republicans, just two seats short of Senate control, are expected to spend tens of millions on attack ads painting him as a Washington insider tainted by lobbyist cash.

Ousting Tester also would cement a Republican lock on a state that voted overwhelmingly for Republican Donald Trump in the 2020 presidential election.

Tester entered the Senate after selling Montana voters on his authenticity, and the former high school band teacher's message hasn't changed much. He still mingles comfortably with union members, ranchers and veterans, has a record of working on their behalf, and says his heart remains firmly in his sparsely populated state, a vast expanse that spans from the arid Great Plains to the lush forests of the Pacific Northwest.

Still, authenticity is harder to sell when you've become a top Washington fundraiser. He's taken in almost $20 million for next year's election, ranking Tester sixth among Senate candidates nationwide, according to Federal Election Commission data through September. Tester insisted that the money hasn't changed him, that he doesn't even know where it all comes from.

"I can't tell you who's donating to me. Even from within the state of Montana, I can't tell you who donates to me because I don't look at that list," he said in an interview. "It's not important. I trust that those people believe in me and I'm going to continue to do the same job."

His campaign reports reveal abundant lobbyist cash, the kind that rarely comes from people who don't want something, and yet the lawmaker's journey from outsider to fundraising behemoth has largely been one of necessity. With West Virginia Democratic Sen. Joe Manchin's decision against seeking another term, Tester has become a top target for Senate Minority Leader Mitch McConnell, R-Ky., and his massive fundraising operation.

McConnell's aspirations to again be majority leader could get bogged down if a primary fight develops between his anointed candidate in Montana, U.S. Navy SEAL Tim Sheehy, and U.S. Rep. Matt Rosendale, one of the far-right House members who ousted fellow Republican Kevin McCarthy of California as House speaker. Dozens of state lawmakers have encouraged Rosendale to enter. He ran against Tester in 2018 and lost despite a huge push from then-President Trump.

Republican unity next November would narrow Tester's path to victory, especially if he's branded as a Washington insider. As he's gained seniority and influence -- and as election spending nationwide has exploded -- the flood of campaign cash that's flowed toward Tester has left him vulnerable to attack.

The potency of the authenticity issue even within his own party was on display during a recent town hall hosted by Tester in the Democratic stronghold of Butte, where a group of activists pressed him repeatedly to call for a cease-fire in the Israel-Hamas war. The lawmaker, who heads the Senate Appropriations subcommittee on defense, rebuffed their pleas, saying Israel had a right to defend itself against the Oct. 7 attack by Hamas.

That rankled Noah Sohl of Missoula, who said he twice voted for Tester and supported the Democrat's last reelection by making phone calls and registering voters. The nursing student drew a straight line between donations to Tester's campaign by defense industry lobbyists and Tester's opposition to a cease-fire.

After becoming the panel's chairman in 2021, Tester received more than $160,000 in contributions from employees and committees representing the defense industry. The donations came at a crucial juncture for both the defense budget and Lockheed Martin, which benefited from $1.8 billion for the F-35 fighter jet that Tester's subcommittee pushed as part of a military spending package.

Sohl pledged not to help Tester this election if he won't change his stance on a cease-fire. Sohl acknowledged that could benefit Republicans.

"They're all licking their chops over the fact that among his (Tester's) constituents, there's a rising group that don't agree with him," Sohl said. "His big thing is, 'I'm not like those Republicans. I'm a true Montanan just going to Washington to fight for the people who voted for me.' But it seems like he lost his footing."

Tester dismissed any notion that campaign donations sway his vote or that he's fundamentally changed since 2006. He also brushed off the increased pressure on him since Manchin's departure.

Veterans issues resound in Montana, which has the second-highest percentage of veterans in the U.S. among the adult civilian population, according to the U.S. Census Bureau. Tester chairs the Senate Veterans' Affairs Committee.

"I take my cues directly from the veterans of this state," Tester told the assembly at Bigfork High School.

In the front row sat Terry Baker, who served in the Vietnam War and voted against Tester the first time he ran. Tester's backing of veterans converted Baker into a supporter. He said the lawmaker remains the only Democrat he has ever voted for.

"He's been a tremendous asset for all the veterans' groups," said Baker, 72, of Kalispell. "The fact that there are a tremendous amount of veterans in Montana, that will help Tester out."

Montana itself has changed significantly since Tester came to office. There's been an influx of newcomers from Arizona, Washington state, California and Texas. Farmland is yielding to subdivisions even as cities such as Bozeman and Missoula have housing crises.

The state's politics have lurched rightward. When Tester entered the Senate, Democrats held almost every statewide elected office in Montana, from governor, secretary of state and attorney general, to two of the state's three seats in Congress. Since the 2020 election, that's down to Tester's seat.

Republican state lawmakers maneuvered unsuccessfully to hobble his chances for a fourth term this spring. They proposed election rule changes that would have allowed only the top two candidates to advance from next year's Senate primary. That likely would have kept third-party candidates off the general election ballot and could have tipped the election for Republicans.

Past races for Tester's seat were close enough some Republicans blamed third-party candidates for the Democrat's victories. Concern that could happen again can't be discounted. Montana Libertarian Party Chairman Sid Daoud announced Monday that he's entering next year's Senate race, raising Republican fears of a third-party spoiler.

Tester rode to office on the unpopularity of the Iraq war and a specter of scandal that plagued his predecessor, three-term Sen. Conrad Burns, over the Republican's close ties to "super-lobbyist" Jack Abramoff. Abramoff was jailed for conspiracy and fraud. No charges were filed against Burns, a former cattle auctioneer who dismissed criticism over the matter as "old political hooey."

Challenges to Tester's authenticity dogged him during the 2018 election cycle, when he ranked for a time as the top recipient of lobbyist donations among members of Congress. He currently ranks second with $407,000 in contributions from lobbyists, putting him just behind Washington state Democratic Sen. Maria Cantwell, according to the research group OpenSecrets.

Those direct contributions do not include millions of dollars expected to be spent on the race from outside groups, including McConnell's formidable operation and comparable Democratic organizations.

So far, there is nothing to indicate that money flowing Tester's way swayed his decision-making or that he did anything wrong. Still, Republicans have highlighted the ties to lobbyists in an ad campaign launched earlier this month that declares "after nearly two decades in Washington, Jon Tester has changed."

Tester invited anyone who think he's changed to come out and "pick rock" at the farm near the small town of Big Sandy that he runs with his wife, Sharla. He said he's still cognizant that in Montana, every connection with voters is vital.

To his way of thinking, that makes authenticity the kind of thing money can't buy.

"This is an eyeball-to-eyeball state," he said.