“He’s running,” said former conservative Sen. Jim DeMint (R-S.C.), who has pledged to help Rosendale. “Of course he’s running.”

Rosendale remains publicly undecided about a Senate bid. But the private steps he’s taking threaten to complicate his party’s carefully laid plans to unseat Sen. Jon Tester, one of the chamber’s most endangered Democrats.

GOP leaders launched a concerted campaign to box out Rosendale and clear the path for Tim Sheehy, a Navy SEAL-turned-business executive who launched a Senate bid in June. Their fear was that Rosendale would prevail in a primary, given his statewide name recognition, but flub a general election against Tester — just as he did in 2018. With a narrow deficit in the Senate, winning in Montana is a linchpin in the party’s broader 2024 strategy.

But Rosendale appears undeterred. He has brought on Caroline Wren, a former Trump campaign aide who helps fundraise for conservative candidates, including Kari Lake in Arizona. And he’s spent the summer attending events in Kalispell, Bozeman and Polson, all of which are represented by Rep. Ryan Zinke (R-Mont.), who has endorsed Sheehy — a sign that his ambitions are statewide.

In recent weeks, he has received encouragement to jump in from Sens. Rand Paul (R-Ky.) and Mike Lee (R-Utah), as well as members of the House Freedom Caucus. And he has secured vows of support from DeMint and members with a slew of allied groups, including Heritage Action and Turning Point Action. Senate Conservatives Fund, a PAC founded by DeMint, is also expected to back Rosendale should he enter tweets over the past two weeksthe race, according to two people familiar with those conversations.

(A spokesperson for Heritage Action said the group does not get involved in GOP primaries and that it has not made plans to support any candidate in the race.)

Several prominent conservatives sent a flurry of tweets over the past two weeks, praising the congressman, including Reps. Matt Gaetz (R-Fla.) and Eli Crane (R-Ariz.).

“The Senate is not going to change the direction of the country unless we have more people like Matt over there,” said DeMint. “So I encouraged him to run. I’m a big supporter of his being there to help change the culture a little bit on the Senate side.”

Speculation about a statewide run heightened when Rosendale attended a fundraiser last week in Kalispell. Local GOP supporters of Rosendale’s placed a piece of tape over one of Rosendale’s campaign signs to black out a line that said “U.S. Congress” — a cheeky push to encourage him to mount a Senate run, according to a person familiar with the event.

“Rep. Rosendale is the clear choice among Montana voters. He has their overwhelming trust and support, should he decide to run,” said Aashka Varma, a top aide to the congressman.

Varma also took a shot at Sheehy, previewing potential attack lines on diversity, equity and inclusion programs and environmental, social, and governing investing — two contentious issues with the party’s right flank. “The people of Montana,” she said, “will not elect a candidate who supports Leftist ESG and DEI provisions and sending American troops into Ukraine.”

The two men have, at times, been allies. Sheehy hosted various campaign events for Rosendale over the years at his home, according to a person familiar with those gatherings.

Sheehy, an aerial firefighter with great personal wealth, has enjoyed a wide array of support since entering the race. Sen. Steve Daines (R-Mont.), the chair of the National Senatorial Campaign Committee, took the unusual step of endorsing him and publicly nudging Rosendale to remain in the House. A slew of senators, including Minority Whip John Thune (R-S.D.), followed his lead in a clear and coordinated effort to send Rosendale a message: stay out.

“I really like Matt Rosendale, which is why I am encouraging him to build seniority for the great state of Montana in the House and help Republicans hold their majority,” Daines told POLITICO in June.

The impact of any eventual primary will be felt far beyond Montana’s borders. Republicans need to capture two and potentially just one seat to reclaim the Senate. Their strongest targets lie in West Virginia and Montana.

But the real blow for Rosendale came last month when David McIntosh, the president of the influential Club for Growth, signaled that the group was not certain it would back the congressman in a Senate race, despite supporting him in 2018.

The congressman has said publicly that he spoke with McIntosh and that the Club president relayed that he was “misquoted.” But McIntosh has not publicly walked back his remarks. According to a person familiar with that conversation, McIntosh did not tell Rosendale that he was misquoted when the two men spoke.

Questions about the Club’s support of Rosendale began late last month. Speaking to a group of reporters, McIntosh praised Sheehy as an impressive candidate and said his group had not decided what to do if Rosendale decided to run.

“If he does, we’re going to take a close look at that race and figure out what the best answer is,” McIntosh said.

At a similar dinner in February, McIntosh adopted a very different posture: “If he decides to run, we’d want to support him again,” he said of Rosendale.

The Club has an impressive collection of wealthy donors and is able to funnel money into key races. It already plans to boost Rep. Alex Mooney in another potentially contentious GOP primary — the Senate race in West Virginia — and has helped collect $13 million. It’s not clear what it would spend — or if it would spend — in Montana.

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