BILLINGS, Mont. (AP) — Montana Republican U.S. Rep. Matt Rosendale plans to run for U.S. Senate, upending a race in which many national GOP officials already coalesced around a different candidate as they seek to unseat three-term Democrat U.S. Sen. Jon Tester.

Rosendale’s intentions were disclosed Wednesday by two people close to the congressman. They spoke on condition of anonymity because they were not authorized to publicly release details of the announcement expected at a state GOP gathering this weekend.

The move sets up a bitter June 4 primary battle between the firebrand conservative Rosendale and former Navy SEAL Tim Sheehy, whose backers include Montana U.S. Sen. Steve Daines, chair of the National Republican Senatorial Committee, and other prominent Senate Republicans.

GOP leaders are eager to unseat Tester as they try to wrest control of the Senate from Democrats, who hold a slim majority and will have several vulnerable incumbents on the 2024 ballot, including Tester.

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Rosendale already lost to Tester once, in a 2018 bid for the Senate. The Republican was serving as state auditor at the time. He was strongly backed by then-President Donald Trump, who had a personal grievance with Tester and visited Montana repeatedly after Tester derailed Trump’s Veterans Affairs nominee.

Rosendale had been hinting at a possible run for months, frustrating some Republicans including members of his own Congressional delegation who wanted the party to unite behind Sheehy.

“It highlights divisions within the Republican Party,” said political analyst Jeremy Johnson at Carroll College in Helena. “Certainly they’ll both be spending money in the primary campaign. If it gets really negative, that’s certainly a benefit for the Democratic candidate.”

Despite Sheehy’s allure to national party leaders, Rosendale maintains significant support within Montana. Dozens of GOP state lawmakers last summer encouraged him to run. In the House, he represents a solidly Republican district covering most of eastern and central Montana and easily fended off his challengers in 2022, meaning he would have faced a relatively easy path to re-election to a third term.

In Washington, Rosendale is among the House’s most hard-right conservatives and a member of the House Freedom Caucus. He banded with seven other members of his party in October to oust Republican Speaker Kevin McCarthy.

He supports Trump, voted against certifying the 2020 election, and cosponsored legislation with Republican U.S. Rep. Matt Gaetz to defund Jack Smith’s investigation into Trump’s alleged storage of classified documents at Mar-a-Lago.

Sheehy entered the race last June and quickly picked up support both from GOP officials in Washington and some leading Republicans in Montana such as Gov. Greg Gianforte and Rep. Ryan Zinke. He runs an aerial firefighting company, Bridger Aerospace, which he founded near Bozeman. He moved to Montana in 2014 after growing up in Minnesota.

This is his first run for public office, meaning he has no voting track record that could be used against him during the campaign. A spokesperson for Sheehy declined to comment on Rosendale’s plan enter the race.

Tester is a farmer and former state lawmaker who was first elected to the Senate in 2006 with an upset victory over three-term Republican incumbent Conrad Burns. The Democratic moderate won his next two contests also by narrow margins, including a 3.5-percentage-point victory over Rosendale, who was dubbed “Maryland Matt” by Democrats playing up the Republican’s out-of-state origins.

Money from outside groups drove up overall spending in that race to more than $60 million, shattering prior records for Montana elections. Advertising for the 2024 race already has begun to flood Montana’s airwaves.

Montana has veered sharply right politically since Tester first took office. He is now the only Democrat holding statewide office in the Treasure State, where the Legislature and governor’s office also are in Republican hands.

Democrats in Montana for months have relished the notion of a Sheehy-Rosendale matchup. They hope the two Republicans will undercut each other — and their party’s standing among voters — as they fiercely compete to win the primary.

“Buckle up for the battle of the out-of-staters,” Montana Democratic Party spokesperson Hannah Rehm said Wednesday. “Montanans are going to see how out of touch Maryland Matt and Transplant Tim are with our state.”