WASHINGTON — Jeff Sessions, the former senator from Alabama whose tumultuous tenure as President Trump’s attorney general lasted less than two years, plans to announce on Thursday that he will enter the race to reclaim his old seat in 2020, a Republican official said.
Mr. Sessions has remained largely out of the public eye, and has been effectively exiled from Republican politics, since he was forced out of the Trump administration last November. He had repeatedly clashed with the president over his decision to recuse himself from the Justice Department’s investigation into Russian meddling in the 2016 election.
Mr. Trump relentlessly attacked Mr. Sessions both in public and in private, calling him “scared stiff” and his leadership “a total joke,” among other insults, ultimately forcing him to resign. By choosing to run for office now, Mr. Sessions risks reigniting attacks from his former boss, who could undermine his standing among the Republican voters he needs to win next year’s crowded primary election on March 3.
Mr. Trump, for his part, continues to blame Mr. Sessions for the two-year Russia probe, and last weekend he repeatedly denounced Mr. Sessions, saying he was a “jerk” and making it clear Mr. Sessions would not have his support, according to a person briefed on the discussions.
Publicly, however, Mr. Trump has remained silent, although some of his allies have begun expressing their disapproval. Minutes after the news of Mr. Sessions’s decision broke on Wednesday evening, Representative Matt Gaetz of Florida declared on Twitter, “Jeff Sessions returning to the Senate is a terrible idea.”
Mr. Trump won Alabama, a staunchly Republican state, with 62 percent of the vote in 2016, and he remains incredibly popular there.
The race is likely to be one of the most closely watched of the 2020 cycle, and not only because of the lingering tension between Mr. Trump and Mr. Sessions. Many Republicans are bracing for the possibility that Roy S. Moore, the former Alabama chief justice accused of fondling teenage girls in his 30s, could once again become the nominee.
Alabama Republicans nominated Mr. Moore in the 2017 special election to fill the seat Mr. Sessions had held for 20 years before becoming attorney general. But after several women came forward, claiming Mr. Moore had abused them when they were underage, his campaign unraveled. He lost to Doug Jones, who became the first Democrat in a generation to win a Senate seat in Alabama.
Mr. Moore announced in June that he would run again next year.
News of Mr. Sessions’s decision to run startled and dismayed national Republicans, who had hoped that he would step aside to avoid the possibility of being vilified by Mr. Trump — and to spare them the headache of a nationalized race in a state they hope to win back.
Mr. Jones is one of the most vulnerable Democratic incumbents in the Senate, and Republicans have been counting on retaking his seat. Without it, their path to retaining their 53-to-47 majority in the chamber is more challenging.
Mitch McConnell, the Senate majority leader, has been less than thrilled with the idea of a Sessions candidacy, according to two people familiar with his thinking. He views it as a distraction that could end poorly for Republicans — especially if a crowded Republican primary ends with Mr. Moore as the nominee again.
Mr. Sessions had been chewing on the idea of returning to the Senate for several months, making clear to allies and advisers that he was pained at the possibility that his final act in public life could be his contentious 21-month tenure at the Justice Department. He had talked to aides and consultants, had polling conducted and asked friends whether they thought Mr. Trump might warm to him.
But over the last week, Mr. Trump sent word to Mr. Sessions through allies that he would publicly attack him if he ran. And Mr. McConnell recently approached Mr. Trump, asking him whether his feelings about Mr. Sessions might have improved. The president said he was very much still opposed to Mr. Sessions and would make that clear if he had to, according to a person briefed on the discussions.
Several Republican officials thought that had settled the matter — until Mr. Sessions decided to run.
Mr. Sessions’s decision comes just before the Friday deadline to enter the race. While he has massive name recognition, he has less cash on hand in his previous Senate account than Bradley Byrne, a congressman who represents a district near Mobile, public records show — a disadvantage that could prove challenging.
In another sign that Mr. Sessions’s entrance into the race is imminent, Fox News said on Wednesday that he was scheduled to appear on Tucker Carlson’s program on Thursday evening.
Jeremy W. Peters reported from Washington, and Maggie Haberman from New York.