The former attorney general’s rocky relationship with Trump is the main obstacle to his return to the Senate. Sessions was a key Trump ally during the 2016 campaign, but his decision to recuse himself from the investigation into the Russian interference in the 2016 election infuriated the president, who has criticized Sessions harshly in public and in private. Trump previously called appointing Sessions attorney general the “biggest mistake” of his presidency.

Sessions will have to earn back Trump’s support, or at least overcome his ire, to be successful in the crowded primary. Jones is the most vulnerable senator up for reelection in 2020, and a number of Republicans are already running — and competing to be seen as Trump’s biggest supporter.

Several Sessions allies have been calling on him to consider running in recent weeks, including the conservative Club for Growth. Sen. Richard Shelby, the state's senior senator who worked with Sessions for years, has spoken to his former colleague about running again.

"If he runs, I think he would be a formidable candidate," Shelby said earlier this week.

Senate Republicans have not yet picked sides in the primary. Sen. Todd Young (R-Ind.), the chairman of the National Republican Senatorial Committee, said earlier this week he was confident the primary would sort itself out.

"We've got some really strong candidates in that state, and I have no doubt that the people of Alabama are going to choose the most conservative Republican who can actually win a general election," Young said.

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The other Republicans running in the primary include Rep. Bradley Byrne; former Auburn University football coach Tommy Tuberville; Secretary of State John Merrill; state Rep. Arnold Mooney; and Roy Moore, the controversial former judge who lost the special election to Jones amid allegations of sexual misconduct.

The Club for Growth released a poll Wednesday conducted by the firm WPA Intelligence that showed Sessions with a 71 percent favorable rating among Republican primary voters, compared to just 20 percent who viewed him unfavorably. Sessions led the primary field with 36 percent support among GOP voters. Tuberville was second with 23 percent, while Byrne and Moore were tied with 11 percent each. The poll, which was conducted Oct. 29-31, surveyed 511 GOP primary voters with a margin of error of 4.4 percentage points.

Tuberville said in a statement Wednesday that Sessions potentially entering the race was "not a surprise."

"He's been out of the swamp for less than two years, and now he's itching to go back," Tuberville said. "He's another career politician that the voters of Alabama will reject. As attorney general, he failed the president at his point of greatest need."

Byrne, in a statement shortly after news of Sessions plans, did not criticize the former attorney general but said that Trump has been under "constant attack."

"Alabama deserves a senator who will stand with the president and won’t run away and hide from the fight," Byrne said.

Sessions’ campaign would be well-funded: He has $2.5 million left over in his campaign account, more than every candidate except Byrne. But the former senator is not accustomed to competitive elections, and this could be the toughest race he’s ever run. Sessions did not face a primary or general election challenger six years ago.