WASHINGTON — Senator Richard C. Shelby, a shrewd force in the Senate for more than 30 years and a longtime political powerhouse in his home state of Alabama, said on Monday that he would not seek a seventh term.

Mr. Shelby, 86, a onetime conservative Democrat who switched to the Republican Party in 1994, had been hinting that he would not run again next year, and said in an interview that he had decided to bring his time in Washington to a close.

“There is a season for all of this, and I recognize that,” he said. “I had a good run, and I still have a couple of years left.”

“I didn’t mean to stay there that long,” added Mr. Shelby, who was first elected to the House from Tuscaloosa in 1978 and the Senate in a strong year for Democrats in 1986.

His retirement will touch off an intramural scramble for the open seat among Republicans, but Democrats have little chance of picking off a seat in deep-red Alabama, particularly in a midterm election with a Democrat in the White House. He is the fourth Senate Republican to disclose he will not run in 2022, joining Rob Portman of Ohio, Richard M. Burr of North Carolina and Patrick J. Toomey of Pennsylvania.

During his long career, Mr. Shelby achieved the rare feat of becoming chairman of four Senate committees: Banking, Intelligence, Rules and, finally, Appropriations, a perch he used to direct billions of federal dollars back home for space and law enforcement-related facilities as well as transportation projects that provided jobs and other opportunities for Alabamians.

“I have tried to help Alabama on meritorious things,” said Mr. Shelby, saying that he wanted to create conditions where his constituents could have work and educational opportunities. “I have always thought just giving someone a check is short term.”

Mr. Shelby was elected to the Senate in 1986, defeating Jeremiah Denton, a former Vietnam prisoner of war and one of a wave of Republicans elected on Ronald Reagan’s coattails in 1980. Mr. Shelby was considered a “boll weevil,” a group of conservative Southern Democrats who often formed a bloc with Republicans against liberal Democratic initiatives, named for a pest common in the South that destroys cotton crops and is difficult to eradicate.

Mr. Shelby clashed repeatedly with President Bill Clinton after his election in 1992, drawing rebukes from the White House for his opposition, including when he responded to one of Mr. Clinton’s budget proposals with the phrase “the tax man cometh.” Mr. Clinton threatened to move NASA jobs from Alabama to Texas and also limited Mr. Shelby to a single pass for the White House ceremony honoring the University of Alabama’s national championship football team in 1992, a petty gesture but a cardinal sin in Crimson Tide terms.

The morning after the 1994 Republican midterm sweep that gave Republicans control of the House for the first time in 40 years, Mr. Shelby announced with great fanfare that he was switching to the Republican Party. The high-profile move left his Democratic colleagues incensed but kept him on course for a long career in the Senate. Mr. Shelby intimidated would-be challengers by amassing a huge campaign war chest that typically deterred anyone considering a run.

In 2017, Mr. Shelby injected himself into the state’s race for the Senate seat left vacant when Jeff Sessions became attorney general.

In a television interview, Mr. Shelby made clear that he could not vote for the Republican nominee, Roy S. Moore, a former judge who had been accused of trying to establish relationships with teenage girls while he was in his 30s. Mr. Shelby’s rejection of him was considered a factor in Mr. Moore’s loss in the election to the Democrat Doug Jones, who was defeated last year in his bid for a full term.

After Mr. Jones’s 20-point loss to the Republican Tommy Tuberville, the most intense action in Alabama will take place among Republicans, with a crowded primary field likely.

Mr. Shelby has spoken highly of Katie Britt, one of his former aides who now runs Alabama’s most influential business lobby. Representative Gary Palmer or former Representative Bradley Byrne, who ran for Senate but lost in last year’s primary, may also enter the race.

If the party remains on its populist trajectory, however, a candidate more in the vein of former President Donald J. Trump could emerge as the front-runner. Representative Mo Brooks, the hard-right conservative, may be that contender, and on Monday he expressed interest in the opening.

But it was far from certain that he could secure the former president’s endorsement in the primary. Mr. Brooks infuriated some people in Mr. Trump’s inner circle by attempting to shift blame to the White House for his role in organizing the Jan. 6 rally in Washington that turned into a deadly mob attack on the Capitol.

The broader challenge for any Republican in Alabama is that without Mr. Sessions, who had become a nemesis of Mr. Trump’s by the time he ran to reclaim his old Senate seat last year, in the field this time, the former president may be reluctant to intervene. Alabama is one of the few states where he had previously offered an endorsement in a contested primary only to see his candidate defeated, as former Senator Luther Strange was by Mr. Moore in 2017.

Mr. Shelby will exit as his state’s longest-serving senator. Though he has had some health problems in the past, he said he is “spry” these days.

“Although I plan to retire, I am not leaving today,” he said in a statement. “I have two good years remaining to continue my work in Washington. I have the vision and the energy to give it my all.”

Jonathan Martin contributed reporting.