Sen. Roy Blunt (Mo.) — the No. 4 Senate Republican leader and an experienced Capitol Hill dealmaker — said Monday that he will not seek reelection next year, becoming the fifth longtime GOP senator to bow out of the midterm election.

Although Blunt’s seat is likely to remain in Republican hands, his retirement is certain to spark a chaotic primary campaign that could pit a controversial former governor against several Missouri officials.

That race could ultimately nominate a candidate much more ideological and confrontational than Blunt, 71, who has spent 24 years in the House and the Senate balancing party loyalty as a longtime member of the GOP leadership with considerable skills as an inside operator able to cut high-stakes deals with Democrats on must-pass bills.

Blunt found his cordial, transactional style increasingly out of step with the mood of his party’s base — especially after the election of President Donald Trump, whom Blunt sometimes gently criticized, particularly on matters of foreign policy. But he gave little indication that he was seriously considering retirement as he voted consistently in line with Trump — at least until Jan. 6, when he broke from scores of congressional Republicans, and Missouri’s junior Republican senator, Josh Hawley, to reject challenges to Trump’s November loss.

His decision creates immediate headaches for Republican political strategists, who face a fifth open seat in next year’s election, another intraparty contest that could complicate the GOP’s quest to retake the majority.

Sens. Richard Burr (N.C.), Rob Portman (Ohio), Richard C. Shelby (Ala.) and Patrick J. Toomey (Pa.) already have said they will not seek reelection. Sens. Charles E. Grassley (Iowa) and Ron Johnson (Wis.) have yet to announce their intentions.

Bill Hoagland, a senior vice president at the Bipartisan Policy Center and a former Senate GOP aide, said Blunt’s departure heralds a continued drift rightward for Senate Republicans — and a possible trend toward even more confrontational legislative tactics.

Blunt, Portman, Shelby and others, he noted, have reliably voted for bipartisan deals to fund the government and reauthorize key federal programs that other Republicans have shunned.

“Their votes will be missed,” he said. “And if they are not there, this is going to make it even harder to deal with some of the critical issues that we are going to be facing going forward.”

For the past six years, Blunt has managed one of the largest and most sensitive portfolios on the Senate Appropriations Committee — overseeing hundreds of billions of dollars in yearly health-care and labor spending that routinely becomes the focus of pitched policy battles over abortion and other issues.

He also serves on the Senate Intelligence Committee, whose work is among the most bipartisan on Capitol Hill, and recently presided over the congressional committee that organized President Biden’s inauguration.

Senate Minority Leader Mitch McConnell (R-Ky.) on Monday called Blunt’s decision “a loss for the Republican conference and the entire Senate.”

“In just 10 years in this body, he’s quickly become a true leader, a policy heavyweight, and a driving force behind both key conservative victories and essential bipartisan work,” McConnell said in a statement. “I’m very sorry he’ll be stepping away but am glad the country has two more years to keep benefiting from his talent.”

In a two-minute video announcing the surprise decision Monday, Blunt cited his “practical sense of getting the job done” in looking back on his congressional service, highlighting his role in funding medical research and improving mental health care.

“After 14 general election victories — three to county office, seven to the United States House of Representatives, and four statewide elections — I won’t be a candidate for reelection to the United States Senate next year,” he said, adding: “There’s still a lot to do, and I look forward to every day, this year and next year, as I continue to work for you in the Senate.”

Sen. Gary Peters (D-Mich.), chairman of the Democratic Senatorial Campaign Committee, told reporters Monday that Blunt’s decision — joining the other GOP senators who have opted for retirement — “speaks volumes about what is happening in the Republican Party right now.”

“That certainly means that Republicans are viewing their party as in trouble, as one that is going to have a real difficult year next year,” he said.

But Sen. Rick Scott (Fla.), the chairman of the National Republican Senatorial Committee, cited Trump’s 15-point margin of victory in Missouri last year in guaranteeing that Republicans would retain the seat.

That analysis, however, could gloss over some of the complications that an open seat could prompt.

Over the past two decades, Missouri has transformed from a perennial swing state into solid Republican territory as White, working-class voters have moved sharply away from the Democratic Party. But GOP strategists fear that a divisive Republican nominee could run into trouble against a talented Democrat — with much of the anxiety surrounding a potential comeback attempt by former governor Eric Greitens, who had previously floated a possible primary challenge to Blunt.

Greitens, a former Navy SEAL, resigned in 2018 under the threat of impeachment after he was criminally charged in connection with an alleged blackmail scheme that targeted a woman with whom he had an extramarital affair. Greitens admitted the affair but denied committing blackmail and other allegations of abuse. The charges were dropped. Last year, he accused his political enemies of fabricating the scheme to drive him from office.

Any number of other Missouri Republicans, however, could enter the race. GOP strategists involved in the state think that Attorney General Eric Schmitt, Secretary of State John R. “Jay” Ashcroft, Lieutenant Gov. Mike Kehoe, Reps. Jason T. Smith and Ann Wagner, and Tim Garrison, a former U.S. attorney, could all mount credible runs, among others.

Ashcroft and Wagner issued statements Monday saying they would explore entering the race, while Greitens — in an appearance on the “War Room” podcast hosted by former Trump aide Stephen K. Bannon — did not say whether he intended to run for the seat while hailing Blunt’s retirement.

“The fact is, [Trump] needs fighters and he needs people who are willing to fight behind the America-first agenda, and the fact is, Sen. Blunt . . . threw his lot in with Mitch McConnell,” he said.

Meanwhile, two of the best-known Missouri Democrats said they would pass on 2022 runs. Former senator Claire McCaskill, who lost her seat to Hawley in 2018, tweeted Monday that she “will never run for office again.”

And Jason Kander, a former Missouri secretary of state who came within three points of beating Blunt in 2016, said in a tweet that his previously announced decision not to run stood in light of Blunt’s decision.

One Democrat who already has entered the race, state Sen. Scott Sifton, welcomed Blunt’s announcement Monday.

“Missourians have an opportunity to vote for better leadership than they’ve been getting from our two senators,” said Sifton, who represents a suburban St. Louis district. “We cannot double down on the dangerous Josh Hawley approach of undermining democracy and dividing Missourians.”

Sean Sullivan contributed to this report.