Rep. Matt Rosendale speaks at a press conference on the debt limit and the Freedom Caucus's plan for spending reduction at the U.S. Capitol on March 28, 2023 in Washington, DC.

Watch CNN’s Manu Raju report on the Montana Senate race on “Inside Politics Sunday” at 11 a.m. ET on December 24 on CNN.

Rep. Matt Rosendale was fundraising at Mar-a-Lago earlier this month when he ran into former President Donald Trump on his way back from the golf course.

Their encounter — described by sources as brief and polite — soon was promoted with photos by the Montana Republican on social media as he praised Trump as “the best president of my lifetime” and said the Senate needed “strong conservatives” to enact Trump’s agenda.

But what frustrated advisers close to the former president: Rosendale still had not endorsed Trump in 2024 but was using their encounter to promote his potential Senate bid. And it wasn’t until three days after their interaction in South Florida that Rosendale finally endorsed the former president.

As Rosendale is now taking steps to mount a Senate primary run and promote himself as a MAGA warrior, top Republicans are hoping to shut down any hope that the conservative hardliner could use Trump as a springboard to the nomination.

GOP leaders are closing ranks behind Republican Tim Sheehy, whom they believe stands the best shot at defeating Democratic Sen. Jon Tester in a race central to the battle for the next Senate majority. And they view Rosendale, who lost by 3 points against Tester in 2018, as the kind of unreliable general election candidate who has thwarted their chances at the Senate majority in a number of recent election cycles.

Sen. Jon Tester walks back to his office after a vote on Capitol Hill on Tuesday, May 16, 2023, in Washington, DC.

But if Rosendale runs against Sheehy, he would have a serious shot at winning the GOP primary as he bills himself as the rock-ribbed conservative in the race — especially if he stays in Trump’s good graces.

So Republican leaders want it to be abundantly clear: It was Sheehy who endorsed Trump months ago, while Rosendale waited until mid-December — underscoring Trump’s enduring sway in primary races as they hope that his desire for loyalty will trump all else.

“Well, Tim Sheehy endorsed President Trump in April. It’s a pretty late endorsement for Matt Rosendale,” Sen. Steve Daines, the Montana Republican who chairs the National Republican Senatorial Committee, which is backing Sheehy’s candidacy, told CNN. “This time, he sees the inevitable that President Trump will be the nominee. I like Matt Rosendale. I hope he stays in the House and builds seniority.”

Trump had privately told Rosendale over the summer that he wouldn’t win his endorsement if he ran for the Senate, according to sources familiar with the call.

But if Trump were to ultimately endorse Sheehy, that move could undermine Rosendale, who has been a controversial figure in Washington and joined seven other Republicans and all Democrats to vote for the ouster of Kevin McCarthy from the speakership.

If Trump stays neutral, as some of his advisers believe he may, it could give an opening to Rosendale, who is likely to face a deluge of attack ads from groups aligned with GOP leaders but who might benefit from outside organizations now pummeling Sheehy on the airwaves.

In an interview, Rosendale defended the timing of his endorsement of Trump, which came roughly a month before the Iowa caucuses.

“I just thought it would be more beneficial for the endorsement to come in at this time,” Rosendale said. “There’s a lot of people that rushed out at the beginning of it, and I didn’t need to be part of a big crowd. I just wanted to make sure that it stood out as a single endorsement, and it would have more impact.”

Asked if the former president was annoyed at his late endorsement, Rosendale said, “He didn’t convey that message to me” when they spoke at Mar-a-Lago.

GOP primaries loom over Senate map

What Rosendale decides to do remains one of the most critical questions in the 2024 Senate election cycle. If he runs, as he’s indicating he will, it could be a boon for Democrats, who are hoping that a handful of contested GOP primaries across the country could help them cling to their majority as they face one of the most daunting maps in years, having to defend 23 seats compared to 11 for the GOP.

Democrats only have two pickup opportunities — both in GOP terrain — in Florida and Texas, while they are defending Democratic seats in three states that Trump won: Ohio, West Virginia and Montana, which Trump carried by 16 points in 2020.

“We prefer a contested primary,” said Michigan Sen. Gary Peters, the Democratic Senatorial Campaign Committee chairman.

But in Montana, Peters wouldn’t characterize Rosendale as a weaker candidate, arguing that Sheehy has “vulnerabilities that will become quite apparent as the campaign goes on.”

“I don’t think it matters one way or the other (whether it’s Rosendale or Sheehy). Tester has already beaten him once. He can beat him again,” Peters said of Rosendale.

As Rosendale weighs a run, there are lingering questions about whether he would be able to keep pace with the campaign apparatus and fundraising of Sheehy, a military veteran and head of an aerial firefighting company, who has been campaigning for months.

But Rosendale may get an unexpected boost.

Then-President Donald Trump and Matt Rosendale during a campaign rally at Four Seasons Arena in Great Falls, Montana, in 2018.

Sheehy already has come under intense attack from a mysterious super PAC, the Last Best Place PAC, which has dropped $1.6 million in ads bashing the Republican candidate. The newly formed group doesn’t have to disclose its donors until next year, and a treasurer for the group, David Lewis, wouldn’t reveal which strategists were driving the ad campaign.

A spokesperson at Senate Majority PAC, the high-spending super PAC linked to Senate Majority Leader Chuck Schumer, declined to comment when asked if they were behind the effort. (Peters said the DSCC would not meddle in any GOP primaries.)

“Last Best Place PAC is clearly run by Chuck Schumer’s allies, but it’s not surprising they want to hide that fact,” said Mike Berg, spokesman for the NRSC.

Asked if Democrats should be pushing Rosendale to get into the race, Tester warned: “Be careful what you wish for,” noting Rosendale has more political experience than Sheehy.

But Tester told CNN that it wouldn’t “make a lick of difference” which candidate he faces.

Top Republicans flatly disagree.

Unlike in 2022, when the NRSC — under different leadership — stayed out of primary races entirely, Daines has taken a heavier hand, seeking to prop up the candidates he believes have the best chance at winning in November, including Sheehy. And he has strategically sought to keep Trump on the same side, ensuring they don’t clash over candidates in primaries and hurt their prospects next November.

GOP Sen. John Cornyn of Texas, a former NRSC chairman, said that GOP primaries remain a major challenge in 2024 — and that Montana could be one of them.

“It’s an old lesson that we forget sometimes, which is winning elections is more than just about winning the primary,” Cornyn said. “And I think that’s the concern, because there are some people who are capable of winning a primary, but they’re not capable of winning the general election because they refuse to broaden their appeal to a majority of the voters sufficient to actually win a general election.”

Asked about Rosendale, Cornyn told CNN: “I think (Daines’) judgment is Rosendale can’t win and that Sheehy can. And you don’t get any points for coming in second in this business.”

Daines said that he believed that — based on polls he’s reviewed — that “Sheehy would beat” Rosendale in a potential primary.

“So I hope Matt stays in that House seat,” Daines said. Asked why Rosendale wasn’t listening: “You’re going to have to ask him.”

In a statement to CNN this week, Rosendale pushed back on Daines and said, “I have always stood by” Trump, whom he called “the most effective” US leader in his lifetime while contending he would be Trump’s “strongest ally” if he returns to the White House in 2025. Rosendale criticized Sheehy as “Mitch McConnell’s hand-picked candidate” and sought to tie him to GOP presidential hopeful Nikki Haley.

Sheehy’s campaign declined to comment on Rosendale.

But in a December 13 memo to donors obtained by CNN, the Sheehy campaign said that his seven-figure ad campaign across the state has been paying dividends, turning around a potential primary that Rosendale appeared to be winning handedly to one where their polling shows the primary race has “shifted to favor” Sheehy.

Trump weighs staying neutral

Unlike in 2022, Trump has taken a more hands-off approach to the 2024 congressional races, a decision many Republicans support after a number of Trump’s preferred candidates collapsed in the general election last year.

Trump has privately told many of his advisers and allies that he believes he made too many endorsements during the midterm elections last year and is planning to stay out of many of the upcoming 2024 congressional fights, three sources familiar with the discussions said. That decision is in part due to his preoccupation with his own campaign, the sources said, but he also doesn’t want to alienate key blocs of conservative voters by engaging too deeply in Republican primaries.

Trump’s more hands-off approach has been welcomed by many of his closest advisers. But it has also frustrated a series of congressional candidates eager to clinch Trump’s support, sources familiar with his lack of endorsements told CNN — especially if he maintains his commanding lead in the polls and goes on to secure the GOP presidential nomination.

Many people close to Trump had anticipated the former president would also be interested in wielding his influence in Montana. But Trump has so far shown little interest in throwing his support behind either candidate, Trump advisers and allies told CNN.

“It’s natural that this is the type of race he’d actually get involved in, given the stakes,” a Trump campaign adviser told CNN. “But the president is very decisive on these things, and there’s something holding him back on this one,” the adviser added.

Some of Trump’s advisers viewed Rosendale’s decision to publicize his December encounter with the former president as an attempt to use Trump’s name and image to bolster his own political career — something they find particularly loathsome, two people close to Trump told CNN.

But Rosendale insists there are no issues between him and Trump.

Matt Rosendale refuses to talk with former President Donald Trump on a phone being offered by Marjorie Taylor Greene during the last moments of a contentious debate on the fourth day of voting for a new speaker of the House on January 7, 2023, in Washington, DC.

Rosendale, who was photographed in January refusing to take Trump’s phone call on the floor as McCarthy was trying to win the speakership, would not say if he asked for the former president’s support for a Senate bid when they spoke at Mar-a-Lago.

“I will never discuss my conversations with the president — with any media,” Rosendale told CNN.

CNN’s Morgan Rimmer contributed to this report.