Now, Sessions — who announced his candidacy Thursday night in a news release posted on his website — faces the challenging task of winning back the president’s support or becoming one of the only Republicans in the past three years to win a GOP primary without Trump’s backing. If successful, he would face Sen. Doug Jones, the Democratic incumbent, who is widely viewed as the most vulnerable senator on the ballot next year.

A handful of Sessions former colleagues are ready to endorse his bid, a show of strength in a crowded Republican primary field. Sen. Richard Shelby, Alabama’s senior senator, has said he would back Sessions. Sens. Roy Blunt of Missouri and John Barrasso of Wyoming, both members of GOP leadership, also told POLITICO on Thursday they would back their former colleague. Other endorsements are expected, as well.

But most of Sessions’ former colleagues are remaining on the sidelines for the crowded primary, and the National Republican Senatorial Committee will remain neutral.

“I guess I’d rather go into a Republican primary with the president’s support than without it,” said Sen. Kevin Cramer of North Dakota. “But Jeff Sessions is iconic, and obviously, he’ll be a player right out of the chute.”

South Carolina Sen. Lindsey Graham, who won’t endorse in the primary, said: “I think the whole campaign is going to be around what President Trump said about Jeff Sessions. I think Jeff knows what he’s getting into, and we’ll leave it up to the voters in Alabama.”

Sessions flew back to Washington on Thursday from Chicago, where he gave a speech at Northwestern University, in time for an interview with Tucker Carlson on Fox News. Carlson is a longtime Sessions friend, and his nightly program draws a wide conservative audience — including the president.

“I was there for the Trump agenda every day I was in the Senate, no doubt about it,” Sessions said in the interview, during which he repeatedly praised Trump and portrayed himself as a top ally of his agenda. He also took several shots at Republicans currently in Congress, saying some remained “standoff-ish” with the president and “haven’t been pushing hard enough to advance the Trump agenda.”

Sessions said he did not regret recusing himself from the Russia investigation — a decision that still infuriates Trump. But he did push back on the impeachment inquiry Trump faces in the House, saying that the president has faced a “continuous political attack” but that Trump “conducted himself in this matter within the law” in his dealings with Ukraine.

The interview and his launch video — in which he praised Trump and did not mention his own Senate campaign — appeared to be an effort to repair the damage of his fractured relationship with Trump and portray himself as a willing defender of the president.

Sessions has been talking privately with two Trump political lieutenants, Bill Stepien and Justin Clark. At Sessions’ request, the two took the temperature of the White House to see how the president would respond. White House senior adviser and Trump son-in-law Jared Kushner was also involved in back-channel conversations with the Sessions team.

Sessions has tapped two Republican strategists, Curt Anderson and Timmy Teepell, to oversee his campaign, according to two people familiar with the choice. As he deliberated over whether to enter the contest — discussions that intensified after Labor Day — Sessions reviewed polling that he had conducted. Among the questions he was forced to ponder was how a Trump assault would affect his prospective candidacy.

Sessions has spoken to several of his colleagues in the months leading up to his decision, several of whom encouraged him to run and others who were pointed in their advice on what he would face during the grueling campaign.

Texas Sen. John Cornyn recalled telling Sessions: “Your life is going to be very difficult unless you work out some sort of reconciliation with the president.” Cornyn added that he hoped the two would reconcile and assumed that Sessions would be the nominee, given his popularity in the state.

The other candidates in the race are Rep. Bradley Byrne, former Auburn University head football coach Tommy Tuberville, Secretary of State John Merrill, state Rep. Arnold Mooney and Roy Moore, who lost the 2017 special election amid allegations of sexual misconduct.

Sessions reached out to the other candidates in the race this week after making his decision to run, according to multiple Republican sources familiar with the conversations. Merrill confirmed that he and Sessions spoke Wednesday, though he declined to discuss the substance of the conversation. Sessions and Byrne spoke Thursday, according to sources familiar with the conversation, and the former attorney general also reached out to Mooney.

“If Sen. Sessions chooses to get in the race, that obviously changes the dynamics of the race completely,” Merrill said in an interview Thursday. He added that it would be “extraordinarily difficult for him to have a successful campaign at the level he’s accustomed to if the president decides that he does not want Sen. Sessions to return to Washington.”

The other candidates have already signaled a willingness to attack Sessions directly. Tuberville put out a video shortly ahead of Sessions’ announcement showing news clips of Trump criticizing his former attorney general. “Pres. Trump said it best when he called Jeff Sessions ‘a disaster’ as AG and an ‘embarrassment to AL.’ The career politicians have let us down,” Tuberville tweeted .

Jones brushed off any concerns about Sessions entering the race, saying of the GOP primary: “They’re the ones that are running around with their hair on fire. It doesn’t affect anything that we’re doing.”

If elected, Sessions would return to a chamber he occupied for two decades. But not all of his former colleagues are racing to back him up. Senate Majority Leader Mitch McConnell wouldn’t comment about Sessions directly on Thursday but said he wanted to win Alabama and “whoever the nominee is, we’ll be behind.”

Senate Majority Whip John Thune of South Dakota said that it was up to “the people of Alabama” to decide who the Republican nominee will be, but that if Sessions “ends up prevailing in that race, we’ll look forward to having him as a colleague.”

Sen. Todd Young, chairman of the National Republican Senatorial Committee, said he didn’t expect to take sides in the primary but was confident in Republicans’ position in the general election.

Other senators voiced concern about what a crowded field could mean. Kansas Sen. Pat Roberts, who doesn’t plan to endorse anyone in the primary, said he hoped the party could avoid a brutal fight.

“I just hope we don’t get into a bad situation and then lose the general,” Roberts said. “If you have a bitter primary fight, sometimes that doesn’t work out very well.”

John Bresnahan contributed to this report.