HELENA — Montana Republicans gathered in a hotel ballroom this weekend aiming to unite ahead of the 2024 election and defeat three-term incumbent Democrat U.S. Sen. Jon Tester. Yet before the party even got underway it was crashed by conservative U.S. Rep. Matt Rosendale, who jumped into the race in defiance of GOP leaders.

Rosendale’s move laid bare deep fissures within the Montana GOP at a time when Republicans can ill afford it. Toppling Tester is a key part of their strategy to take control of the narrowly divided Senate in the November election by targeting vulnerable Democratic seats in Montana, Ohio and West Virginia.

Outside observers and even some Republicans say an intraparty skirmish leading up to Montana’s June primary could undermine those hopes.

Senate Republican leaders — including Sen. Steve Daines of Montana, who leads the National Republican Senatorial Committee — are backing a former U.S. Navy SEAL over Rosendale, who is viewed as too divisive to appeal to the state’s large contingent of independent voters.

Rosendale’s entry into the Senate contest capped months of speculation that the hard-right lawmaker wanted a rematch six years after losing to Tester in 2018.

“I’ve won two elections since then,” Rosendale, 63, told reporters after filing paperwork on Friday to formally enter the race. “And the most important thing is that my name I.D. and my trust factor is elevated dramatically. People know who I am.”

A large group of conservative state lawmakers showed up for Rosendale’s filing and boisterously cheered him on, underscoring his grassroots support in the state.

A few hours later and several blocks away, Montana Gov. Greg Gianforte took the stage in a Helena hotel ballroom to pump up former SEAL and political newcomer Tim Sheehy as the party’s best chance to beat Tester. Behind the Republican governor was a poster with the state GOP slogan, “We’re better, together!”

As Gianforte’s speech concluded a buzz ran through the crowd: Former President Donald Trump had just endorsed Sheehy in a social media post. “He probably heard my speech,” Gianforte quipped as an aide told him of the endorsement.

Rosendale, who Trump backed in 2018 and again in 2020, responded to the setback by reaffirming his fealty to the former president. “I love President Trump,” he said when asked about the endorsement.

Rosendale was among eight conservative lawmakers who ousted House Speaker Kevin McCarthy last year. He suggested during the event in Helena that he hopes to do the same to Republican Senate Minority Leader Mitch McConnell, who Rosendale derided as part of a “uni-party” of Republican and Democrat leaders controlling legislation in Congress.

Sheehy, 38, founded an aerial firefighting company in Belgrade, Montana, that is heavily dependent on federal government contracts. He said in an interview that he decided to enter politics following the chaotic American withdrawal from Afghanistan.

His lack of political experience is a plus, he said, because it means he hasn’t “been contaminated by years in politics.”

“Americans in Montana specifically are really tired of the same people in Washington going back and forth over and over,” he said. “I bring a fresh perspective. I’ve been a small business owner, a job creator for over 400 jobs.”

The federal contracts that helped pay for many of those jobs irk Rosendale’s supporters.

“I see a pretty significant conflict when your livelihood is determined by government contracts,” said Theresa Manzella, a Republican state senator and chairperson of the Montana Freedom Caucus, a group of conservative lawmakers politically aligned with Rosendale.

But state Rep. George Nikolakakos argued that nominating Rosendale would play into the Democrats’ hands.

“Rosendale had his chance in ’18 and lost,” said Nikolakakos, a Republican representing a swing district in Great Falls. “I would say that the people who are going to choose Rosendale and the people who want Rosendale to be the nominee are the Democrats.”

Those Democrats are egging on the division in the GOP Senate race, hopeful it will drain Republican funds and alienate independent voters before the general election. The state Democratic Party responded to Rosendale’s announcement by declaring that the Republican primary would be “a bloody brawl.”

Tester, 67, is a farmer and former state lawmaker who was first elected to the Senate in 2006 in an upset victory over a three-term Republican incumbent. The moderate lawmaker won his next two contests also by narrow margins, including a 3.5-percentage-point victory over Rosendale.

Montana has politically veered sharply right since Tester first took office, leaving him increasingly vulnerable with each election cycle.

Trump beat Biden by 16 percentage points in Montana four years ago and Tester is now the only Democrat holding statewide office there — an abrupt flip from last decade when Republicans faced a similar situation.

The candidates and outside political groups already have spent more than $18 million on advertising in the Senate campaign’s early months. That will quickly ramp up between now and the November election with an additional $95 million in advertising reserved, according to AdImpact, a firm that tracks political advertising.

The heady pace of spending also puts the race on track to be among the most expensive political contests in Montana history, rivaling a 2020 matchup between Daines and then-Gov. Steve Bullock in which more than $118 million was spent.