Matt Rosendale to Run for Senate in Montana, Igniting Republican Fight

Mr. Rosendale and Mr. Gaetz were part of the small band of Republicans who dethroned Representative Kevin McCarthy as House speaker, temporarily shutting down business in the chamber. Mr. Rosendale had objected to installing Mr. McCarthy in the role in the first place, appearing to brush off a phone call on the House floor from Mr. Trump, who had rung up Ms. Greene to urge his support.

Mr. Rosendale also has the backing of Stephen K. Bannon, the former Trump White House strategist who is planning a blitz of campaign events in Montana next month. Mr. Rosendale has been a frequent guest on Mr. Bannon’s “War Room” podcast, which is popular with conservatives.

Mr. Rosendale entered 2024 with $1.7 million in his federal campaign account. That’s about 40 percent of what he spent against Mr. Tester six years ago, but a small fraction of the roughly $44 million that two Republican super PACs have already spent on behalf of Mr. Sheehy, a retired Navy SEAL who founded an aerial firefighting company.

Those groups — the Senate Leadership Fund, which has ties to Senator Mitch McConnell, the Republican leader, and American Crossroads, which was co-founded by Karl Rove — have spent about $22 million each in advertising in the state.

A third super PAC supporting Mr. Sheehy, known as More Jobs, Less Government, has been underwritten by a few wealthy Wall Street executives. Kenneth Griffin gave $5 million, Paul Singer contributed $1 million and Stephen A. Schwarzman chipped in $400,000, according to Federal Election Commission filings. The super PAC has hired Tony Fabrizio, Mr. Trump’s longtime pollster, and Andy Surabian, a strategist who has been involved with all three of Mr. Trump’s presidential campaigns.

Mr. Tester, meanwhile, has kept pace with the surge of Republican spending. His campaign has spent $5 million on an advertising push in the state, and has been aided by roughly $50 million in combined spending from WinSenate and Last Best Place, two Democratic super PACs with links to the Senate Majority PAC, the top outside group for Senate Democrats.

Maggie Haberman contributed reporting.