UPDATE: 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."

He was expected to make a formal announcement on Fox News' "Tucker Carlson Tonight" later Thursday.

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.

PREVIOUS STORY: Former U.S. Attorney General Jeff Sessions intends to announce this week his bid to reclaim his old U.S. Senate seat, two sources familiar with Sessions tell NBC News.

It has been made clear to Sessions that President Donald Trump intends to campaign against him in what is currently a crowded Alabama Senate Republican primary field. Sessions must file his papers to run with the Alabama Republican Party by 5:00 p.m. on Friday night, which he has yet to do.

Sessions, who resigned as Attorney General in 2018 after recusing himself from the federal probe into Russia’s involvement in the 2016 presidential election, would likely face the scorn of Trump, who called his decision to nominate Sessions as attorney general the "biggest mistake" of his presidency.

There have been questions about whether Sessions would be able to construct the campaign infrastructure to compete in a crowded GOP field ahead of the party’s March 3 primary. He has not run in a competitive race since his first campaign for the U.S. Senate in 1996, and the four-term senator lost a network of allies since Trump’s election— some to jobs within the Department of Justice and others, like former White House aides Rick Dearborn and Cliff Sims, who fell out of Trump’s favor. His former campaign manager and state director, Chuck Spurlock, died of cancer in 2013.

Sessions would enter the race with $2.5 million stowed away in his campaign war chest from previous fundraising.

Read the full story at NBC News.