Senator Tester of Montana is entering one of the most closely watched Senate races of the year leading his likely GOP opponent, Tim Sheehy, in a race that has already attracted tens of millions of dollars in investment from both parties. 

A news survey conducted by SurveyUSA found that Mr. Tester is entering his re-election bid leading Mr. Sheehy 49 percent to 40 percent in Montana.

The new data come as Republicans in Montana are consolidating behind Mr. Sheehy as their candidate for November, with Congressman Matt Rosendale, who was expected to run against Mr. Sheehy in the primary, dropping out of the race, citing President Trump’s endorsement of Mr. Sheehy.

“I have long been a supporter of the president, and remain so,” Mr. Rosendale said in a statement. “But I have been forced to calculate what my chances of success would be with Trump supporting my opponent.”

Mr. Sheehy’s campaign has already launched a new ad touting Mr. Trump’s endorsement and the candidate’s military career.

“Trump says Sheehy is a political outsider, who is strong on the border, strong on our military and vets, and strong on the second amendment,” the ad says.

Mr. Sheehy was the favorite of the National Republican Senatorial Committee. Mr. Rosendale had challenged Mr. Tester in 2018, when he was defeated by about 4 points.

In a statement, the chairman of the NRSC, Senator Daines, expressed approval of Mr. Rosendale’s decision to drop out, saying, “I appreciate Matt’s many years of service to Montana.”

“It will take all Republicans working together to defeat Jon Tester in November,” Mr. Daines said.

Mr. Rosendale had lost the final high-ranking Republican who was set to endorse him last week, when Speaker Johnson decided not to.

Mr. Rosendale was among the Republicans who voted to oust Speaker McCarthy last year. It’s not yet clear if he will run for re-election to the House, with a competitive primary to replace him already under way.

Since Mr. Rosendale’s announcement, other Republicans have come out in support of Mr. Sheehy, like Senator Vance, who said, “I’m happy to join President Trump and numerous other conservatives in endorsing his campaign.”

Mr. Sheehy also said of Mr. Rosendale’s decision to drop out: ”I know working together we’ll win this race and defeat Jon Tester.”

In consolidating behind a single candidate, Republicans are gearing up for a race that is widely seen as one of their best opportunities to flip a Senate seat in 2024.

Despite Mr. Trump winning Montana by about 16 points in 2020, and even though Republicans have carried Montana on the presidential level in every election in the past 50 years, except for in 1992, Mr. Tester has proved to be a singular candidate in Senate elections there.

Since narrowly winning election in 2006, Mr. Tester won re-election in 2012 and 2018 by nearly 4 points each time. In 2024, Republicans are already spending millions of dollars to try to keep that from happening again.

The Montana Senate race has already attracted $89.4 million in political ad spending, according to a political ad watchdog, AdImpact. Democratic groups have spent about $26 million there and Republican groups have spent about $44 million.

Mr. Tester is also a top fundraiser on the Democratic side, raising more than $25 million for his re-election bid so far this cycle. Mr. Sheehy has raised just more than $5 million so far, according to FEC filings.