Sessions’ response to the unwelcome news, meanwhile, outlined the box he finds himself in.

Despite Trump’s explicit support for another candidate, Sessions is still running as the true champion and one of the “architects” of Trumpism. He launched his campaign reminding voters about his record and his early endorsement of Trump in the 2016 presidential race, a message that also appeared frequently in his TV ads.

“Nothing the President can do will deter me from supporting this agenda, because my principles, just like my faith, are fundamental to who I am and immovable,” Sessions wrote in a tweet responding to Trump’s endorsement of Tuberville. “We are Alabama. Nobody tells us how to vote or what to do.”

Tuberville has been running as an outsider, promising to be a loyal warrior for Trump in the Senate. The former Auburn University football coach started with an edge in the runoff after his surprising first-place finish in the March 3 primary. He narrowly edged out Sessions — 33.4 percent to 31.6 percent — but fell well short of the 50 percent needed to avoid a runoff for the GOP nomination to face Democratic Sen. Doug Jones.

The Club for Growth, a conservative group which spent heavily against Rep. Bradley Byrne in the primary, isn’t taking sides in the runoff. But a poll conducted last week on the group’s behalf showed Tuberville leading Sessions by 4 percentage. The lead ballooned to 24 percentage points when voters were asked about their vote if Trump were to endorse Tuberville.

“The presidents involved, he’s been involved and Sessions [has] got a tough road ahead of him,” said Sen. Richard Shelby (R-Ala.), who is supporting his former colleague in the race.

Sessions has had to take the fight to Tuberville, hoping to score points on his policy chops by forcing Tuberville into a formal debate before the runoff and attacking Tuberville as a “Florida man”; Tuberville moved to the state after retiring from coaching after the 2016 season at the University of Cincinnati. Tuberville’s campaign has rebuffed efforts to hold a debate during the runoff, and Sessions’ campaign has so far outspent Tuberville on the airwaves since the primary.

An internal poll from Sessions campaign after the primary showed the race tied at 45 percent, with 10 percent of likely voters undecided.

Senate Republicans took an “Anybody but Roy Moore” stance during the primary, hoping to avoid renominating the candidate who lost the 2017 special election to Jones, and they aren’t taking sides during the runoff. Tuberville spoke with Senate Majority Leader Mitch McConnell after the runoff, according to two people familiar with the conversation. But the National Republican Senatorial Committee is not endorsing either candidate.

“We’ve got two strong Republicans who will present strong challenges to Doug Jones. We’re going to win Alabama,” NRSC Chairman Todd Young said.

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Several Alabama Republicans cautioned against writing Sessions political obituary just yet, pointing out that Trump endorsed former Sen. Luther Strange in the 2017 special election in the state, only to see him lose to Roy Moore even after Trump held a rally on Strange’s behalf.

“It’s one thing to send out a tweet. It's another thing to offer support by actually making a visit to the state and by demonstrating your support through multiple tweets or multiple levels of endorsement,” said Alabama Secretary of State John Merrill, who is neutral in the race.

“Obviously the president has been disappointed with Sen. Sessions for some time, but he's been kind of beating around the bush that he might do something related to that race,” he added.

Moore, who lost the special general election in 2017 after he was accused of inappropriate sexual behavior, was never denounced by Trump in the terms Trump has leveled against Sessions. Like Sessions, Moore claimed during his primary runoff against Strange that he was the true Trumpist pick, and Trump was quick to switch his endorsement to Moore when he defeated Strange for the nomination.

One Alabama GOP operative said Trump’s pre-primary rally for Strange happened in a “different world,” before Trump’s endorsement became the powerful intraparty tool it is now. “It’s almost impossible to run against it in a Republican primary now,” the operative said. “It’s death knell.”

In the end, the runoff may come down both to which candidate can create a sense of urgency for their supporters given the quick four-week turnaround after the primary. Byrne, the third-place finisher, got nearly 25 percent of the vote, mostly centered near the Gulf in the heart of his congressional district. Seth Morrow, Byrne’s campaign manager, said the congressman has no plans to endorse in the runoff. But his voters’ preferences could swing things one way or the other if they turn out in droves.

“Do I think this is going to move any Sessions votes away from him? No,” one veteran GOP operative said, referring to Trump’s endorsement. “And the Tuberville votes aren't going anywhere. The piece of the puzzle is the undecided and/or the Byrne voters.”

Marianne LeVine contributed to this report.