"Our freedoms have never been under attack like they are today. We have major party candidates for President campaigning on socialism, confiscating firearms, and closing down churches they disagree with," Sessions said in a press release on his newly launched campaign website. "I've battled these forces my entire life, and I'm not about to surrender now. Let's go!"
In a video accompanying his announcement, Sessions touted his support of the president, saying he's "doing a great job for America and Alabama" and that he has Sessions' "strong support."
Speaking on Fox News shortly after the website was launched, the former attorney general said he would file his candidacy Friday, the deadline to enter Alabama's Senate race.
Sessions has long been speculated as a potential candidate for his old seat, despite his tumultuous relationship with President Donald Trump, after Democrat Doug Jones' upset victory over flawed Republican candidate Roy Moore in 2017. Moore's candidacy was stymied by the emergence of multiple accusations of sexual misconduct in the 1970s, including inappropriate contact with underage women when he was in his 30s, which he denied.
After ultimately losing by more than 22,000 votes to Jones in a major blow to Republicans, Moore announced in June he would again compete for the GOP nomination in 2020. Jones is now considered one of the most vulnerable senators up for re-election in 2020.
Earlier on Thursday, Jones downplayed Sessions' impending announcement, telling SiriusXM's The Joe Madison Show, "Before he challenges me, he's got about six challengers in that Republican primary that are already sniping at him ... I'm going to watch that primary. It's going to be a really divisive primary."
"Now you've got somebody else jumping in there that the president of the United States has said it was the biggest mistake he's ever made by appointing him," Jones added, referencing Trump's interview with NBC in which he called his pick of Sessions as attorney general "the biggest mistake" of his cabinet appointees.
"I would say if I had one do-over, it would be, I would not have appointed Jeff Sessions to be attorney general," Trump said in June, one of his many public attacks against Sessions.
Despite Trump previously saying this, Sessions said on Fox News Thursday that he, "had a great tenure at the Department of Justice in so many different ways," and that "it was an honor to serve" the president.
Sessions entrance into this race comes nearly one year after he exited the Trump administration, which was one day after the 2018 midterm elections and after Trump fired him and replaced him with his chief of staff Matthew Whitaker.
While he is a popular political figure in Alabama, Sessions remains unpopular with his former boss, who carried the state with 62% of the vote over Hillary Clinton in 2016.
"Sessions' fate is almost entirely in Trump's hands," a GOP consultant told ABC News. The source noted that it is likely the president will strongly oppose him, but for Senate Majority Leader Mitch McConnell, the choice is more complicated since the source said McConnell is fond of Sessions and never thought he should've been pushed out as attorney general.
But in launching his campaign, Sessions acknowledged his "ups and downs" with Trump, but only praised the president, and reiterated his place as the first senator to endorse Trump's campaign.
"When I left President Trump's cabinet, did I write a tell all book? No. Did I go on CNN and attack the President? No. Have I said a cross word about President Trump? No," he said in the press release. "I was the first to support President Trump. I was his strongest advocate. I still am. We must make America great again."
It is unclear if the president would embrace his candidacy if he were to secure the nomination, but Sessions said on Fox News Thursday that he hopes Trump will support his candidacy. While he said he hasn't yet been given an opportunity to speak to the president about his campaign, he said he looks "forward to having that opportunity."
"I would like to be able to go to the people of Alabama and tell them with all honesty, I believe in this agenda. I was for this agenda before President Trump announced," Sessions said. "If I return to the Senate, I will -- no senator in the Senate will be more effective in advancing President Trump's agenda than I would be."
During his time at the Justice Department, Sessions was frequently in the spotlight throughout his turbulent time in the Trump administration -- stemming from his recusal from the Russia investigation in March 2017, which Sessions said on Thursday he didn't regret. He was consistently the target of the president's ire as the protracted Mueller investigation loomed over the White House for nearly two years.
The former attorney general is jumping into a crowded field in the Republican primary, which includes Moore, Alabama GOP Rep. Bradley Byrne, former Auburn football coach Tommy Tuberville, state Rep. Arnold Mooney and Alabama Secretary of State John Merrill. The GOP primary is set for March 3, 2020.
Vice President Mike Pence deflected when pressed by reporters about if he'll campaign for Sessions if he were to secure the nomination on Thursday in New Hampshire as he filed for the Republican primary ballot on behalf of the president in the first-in-the-nation primary state.
"We'll let the people of Alabama make that decision," he said.
One of Trump's allies on Capitol Hill, Sen. Lindsey Graham, R-S.C., said that while he is a fan of his former colleague, he called the move "problematic" given the animosity Trump has expressed towards Sessions.
"I like Jeff, I thought he was a great senator. But the whole campaign will be about what Trump said about Sessions. And as you can imagine, that'd be kinda ugly and the president is pretty out there when it comes to the job he did as attorney general," Graham told ABC News on Wednesday. "But I have a lot of respect for Jeff, he was a good senator. We'll see what happens. But you could see that would be problematic. But it would be up to him."
The senior senator from Alabama, Richard Shelby, praised Sessions on Capitol Hill Thursday, saying he would "absolutely" support his bid.
"Jeff Sessions is a friend. I worked with him every day up here for 20 years," Shelby added. "He's a man of integrity and, of course, he'll have to run his own race and that's up to the people of Alabama, but I believe he'll be a formidable candidate."
The last time Shelby spoke of a potential Sessions bid with Trump was two or three months ago, he said, noting that he hasn't spoken to the president recently about it.
"I'll see if President Trump wants to talk about it. I'll see him in Alabama Saturday," Shelby said.
The president is expected to attend a college football game between Alabama and Louisiana State University on Saturday in Tuscaloosa, according to ESPN.
Late last month, amid speculation that he was planning a bid, the president's son, Donald Trump Jr., refused to nudge Sessions towards running, instead hedging on the question in an interview with Yellowhammer News, a conservative website in the state.
Sessions nascent campaign is working with the consulting firm, OnMessage Inc., which has worked with several current and former Republicans in Congress, according to the GOP source.
ABC News' Mariam Khan and Trish Turner contributed to this report.