BOZEMAN, Mont. (AP) — Donald Trump traveled to Montana for a Friday night rally intended to drum up support for ousting the state's Democratic senator, but the former president's plane first had to divert to an airport on the other side of the Rocky Mountains because of a mechanical issue, according to airport staff.

Trump's plane was en route to Bozeman, Montana, when it was diverted Friday afternoon to Billings, 142 miles to the east, according to Jenny Mockel, administrative assistant at Billings Logan International Airport. Trump continued to Bozeman via private jet.

The former president came to Montana hoping to remedy some unfinished business from 2018, when he campaigned repeatedly in Big Sky Country in a failed bid to oust incumbent Democratic Sen. Jon Tester. On Friday, Trump ripped into the three-term senator, mocking him for being overweight and for insinuating he sometimes sided with the former president.

“He voted to impeach me — that guy voted to impeach me,” Trump said of Tester, whom he called a “slob” with “the biggest stomach I've ever seen.”

Trump also invited to the stage Texas Rep. Ronny Jackson, his former White House physician, to further slam Montana’s senior senator. Tester sank Jackson's nomination to be Trump's Veterans Affairs secretary, alleging the doctor drank and used prescription drugs while on duty.

Tester has tried to convince voters he’s aligned with Trump on many issues, mirroring his successful strategy from six years ago. While that worked in a non-presidential election year, it faces a more critical test this fall with Tester's opponent, former Navy SEAL Tim Sheehy, trying to link the three-term incumbent to Democratic presidential candidate Kamala Harris.

Trump kicked off his rally about 90 minutes behind schedule and immediately began lacing into Tester. “We are going to defeat radical left Democrat Jon Tester, he’s terrible,” Trump said. “We’re going to evict crazy Kamala,” he continued, workshopping a nickname on his new rival.

Harris has benefitted nationally from a burst of enthusiasm among core Democratic constituencies, who coalesced quickly around her after President Joe Biden withdrew from the campaign last month. She's drawn big crowds in swing states, touring this week with Minnesota Gov. Tim Walz, her choice to be her vice presidential nominee.

Trump's only rally this week, meanwhile, was in a state he won by 16 percentage points four years ago rather than a November battleground. Facing new pressure in the race from a candidate with surging enthusiasm, Trump on Thursday called questions about his lack of swing state stops “stupid."

“I don’t have to go there because I’m leading those states,” he said. “I’m going because I want to help senators and congressmen get elected.”

He will add on fundraising stops in Wyoming and Colorado.

Trump could be decisive in Montana's Senate race

Friday's rally at Montana State University drew thousands of GOP supporters. Yet the former president's bigger impact could be simply having his name above Sheehy's on the ballot in November, said University of Montana political analyst Rob Saldin.

“There is a segment of the electorate that will turn out when Trump is on the ticket," Saldin said. And that could benefit Sheehy, a Trump supporter and newcomer to politics who made a fortune off an aerial firefighting business.

Republicans have been on a roll in Montana for more than a decade and now hold every statewide office except for Tester’s.

Tester won each of his previous Senate contests by a narrow margin, casting himself as a plainspoken farmer who builds personal connections with people in Montana and is willing to break with his party on issues that matter to them. He's also become a prolific fundraiser.

The race has drawn national attention with Democrats clinging to a razor-thin majority in the Senate and defending far more seats than the GOP this year. Tester is considered among the most vulnerable Democratic incumbents.

For him to win, large numbers of Trump supporters would have to vote a split ticket and get behind the Democratic senator.

Trump’s drive to oust Tester traces back to the lawmaker’s work in 2018 as chairman of the Senate Committee on Veterans' Affairs. Tester revealed past misconduct by Trump’s personal physician, Ronny Jackson, that sank Jackson's nomination to lead the Veterans Affairs Department.

Then-President Trump took the matter personally and came to Montana four times to campaign for Republican Matt Rosendale, who was then the state auditor. Rosendale lost by 3 percentage points.

Tester has positioned himself apart from national Democrats

Before Trump's latest visit, Tester has sought to insulate himself against charges that he's part of the Democratic establishment by rolling out the names of Republicans who support him, including former Montana Gov. Marc Racicot. His campaign highlighted more than 20 pieces of legislation, many dealing with veterans' issues, that Tester sponsored and Trump signed.

Tester also was the sole Democratic delegate from Montana to withhold a vote backing Harris as the party's presidential candidate in the wake of Biden's withdrawal. And when the Democratic National Convention takes place later this month in Chicago, Tester will be back in Montana “farming and meeting face to face with Montanans,” campaign spokesperson Harry Child said.

The last time Tester attended the Democratic National Convention was in 2008. That’s also the last time a Democratic presidential candidate came anywhere near winning Montana, with President Barack Obama losing by just over 2 percentage points.

On Friday, in an interview as he waited for the Trump rally to start, Sheehy dismissed the idea that Tester can survive Montana's swing to the right. “Jon Tester is by 95%-plus in lockstep with the Biden-Harris agenda," Sheehy said. "So I don’t think his attempt to message himself as a moderate is going to work.”

A similar situation is developing in Ohio, where three-term Democratic Sen. Sherrod Brown faces a tough race in a state expected to vote for Trump.

Harris visited Ohio when the two were Senate colleagues to raise money for Brown's 2018 campaign, but Brown has said he has no plans to campaign with her this year. Like Tester, Brown has highlighted legislation he worked on that Trump signed into law.

Friday's rally takes place in Gallatin County, which Tester has become increasingly reliant on over the course of his political career.

He lost the county in his first Senate race, in 2006, but his support has since grown. A substantial margin of victory in Gallatin in 2018 helped push him ahead of Rosendale.

Republican Don Seifert, a former Gallatin County commissioner, said he voted for Tester that year and plans to do so again this year.

Seifert backed Trump in 2016 and said he has continued to support other Republicans, including Montana Gov. Greg Gianforte and Sen. Steve Daines.

“Montanans tend to vote for the person over the party,” Seifert said. “For the state of Montana, Jon is the one that can do what we need.”

But Sheehy says Tester has lost touch with his home state and fallen into step with Democrats in Washington. The Republican said in a message this week to supporters that Tester was “responsible for the rise of Kamala Harris” because he served as chairman of the Democratic Senatorial Campaign Committee from 2015 to 2017, when she was elected to the Senate from California.

Tester has outraised Sheehy by more than three-to-one in campaign donations reported to the Federal Election Commission. However, outside groups supporting Sheehy have helped the Republican make up much of that gap. Spending in the race is on track to exceed $200 million as advertisements from the two sides saturate Montana’s airwaves.

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Associated Press reporters Amy Beth Hanson in Helena, Montana, and Julie Smyth in Columbus, Ohio, contributed to this report.