Former Attorney General Jeff Sessions announced on Thursday that he is jumping into the race for his old Senate seat in Alabama — despite warnings from allies of President Donald Trump that he should sit out the election.

Sessions, who was the first senator to endorse Trump's campaign, suffered a huge falling out with the president over his decision to recuse himself from the federal probe into Russia's involvement in the 2016 election. Trump has said naming Sessions as attorney general was the "biggest mistake" of his presidency, and he has called his leadership of the Justice Department "a total joke."

Sessions resigned at Trump's request exactly one year ago.

In a statement posted to his campaign website, he offered effusive praise of the president despite their rift.

"As everyone knows, President Trump and I have had our ups and downs. But here’s the important part: the President is doing great work for America," he said. "When President Trump took on Washington, only one Senator out of a hundred had the courage to stand with him: me. I was the first to support President Trump. I was his strongest advocate. I still am. We must make America great again."

In an interview on Fox News' "Tucker Carlson Tonight," Sessions said he was "honored" to serve as attorney general.

"We were able to serve, to push the Trump agenda," he said. "I don't regret that."

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Sessions' Senate announcement comes the night before the deadline to file in the hyper-competitive Republican race, which already includes former Auburn University football coach Tommy Tuberville and former state Supreme Court Chief Justice Roy Moore, who was dogged by allegations of sexual misconduct as he lost to Democrat Doug Jones in the 2017 special election to replace Sessions.

Tuberville released a digital ad Thursday that started with Trump saying the one thing he'd like to do over would be naming Sessions his attorney general. Grit PAC, a political action committee supporting Tuberville's candidacy, also released a 15-second online ad calling Sessions a "traitor."

Sessions' participation comes despite his being told behind the scenes and publicly by allies of the president that Trump would campaign against him in the crowded Republican primary field.

"Jeff Sessions returning to the Senate is a terrible idea," Rep. Matt Gaetz of Florida tweeted Wednesday.

Sen. Richard Shelby of Alabama, however, told reporters on Thursday that he'd "absolutely" endorse his former colleague if he got into the race.

"Jeff Sessions is a friend. I worked with him every day up here for 20 years. He's a man of integrity. Of course, he'll have to run his own race, and you know that's up to the people of Alabama, but I believe he'll be a formidable candidate," Shelby said.