MADISON, Alabama — Jeff Sessions of Alabama became the first sitting U.S. senator to endorse Donald Trump on Sunday.

Sessions, donning a red "Make America Great Again" hat, told the crowd that he was throwing his support behind this "movement" because, in his best estimation, "at this time in America's history, we need to make America great again!"

The endorsement comes on the heels of another high-profile backing from New Jersey Gov. Chris Christie. Despite having criticized Trump during his own run for the Republican nomination, Christie threw his support behind the New York real estate mogul, who he said was a longtime friend.

Christie spent Friday and Saturday on the trail traveling with Trump, touting his credentials and slamming rivals, specifically Sen. Marco Rubio of Florida.

While Christie brings an outside-the-beltway gravitas and perspective, Sessions’ backing is the first major sign Trump is earning the faith of true conservatives on Capitol Hill. In an election cycle with Supreme Court nominations in the spotlight, Sessions’ position on the Senate Judiciary Committee bolsters the real estate mogul’s conservative credentials, despite Trump's lack of specifics on what he would look for in a nominee.

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Sessions' position as chairman on the Senate sub-committee on Immigration further legitimizes one of the central pillars of Trump’s candidacy — his plan to build a wall along the southern U.S. border and have Mexico foot the bill.

"I'm becoming mainstream. All these people are now endorsing me," Trump told the sprawling Alabama crowd. And Trump relished the fact that Sessions, who has served in Congress for nearly 20 years, has never endorsed a presidential candidate. "When I get Jeff Sessions, that means a lot to me. That means a lot. That's a biggie, especially since he's never done it before."

Related: Chris Christie Endorses Donald Trump for President

Sessions' endorsement startled the audience but wasn't necessarily surprising given the two men's history.

Sessions first appeared with Trump on stage in Mobile late last summer. He stopped short of endorsing Trump then, but he was an adviser on Trump's immigration policy plan. The Alabama senator has been bullish against comprehensive immigration reform efforts in the Senate, fighting back against proposals such as the "Gang of 8" bill and railing against President Obama’s immigration executive actions.

Sessions praised Trump on Sunday as the man who can finally follow through on years of unfinished promises to fix illegal immigration. "Donald Trump will do it," he announced to cheers.

In September, after Trump appeared at a rally opposing President Barack Obama's nuclear deal with Iran, the two men met on Capitol Hill, joined by Sarah Palin, who has since endorsed Trump, as well. And in recent weeks, one of Sessions' top aides joined the Trump campaign as a senior adviser.

Sessions even seemed to back Trump’s controversial decision to skip a GOP debate in Iowa held just before the caucuses, saying at the time that he respected Trump’s willingness to walk away. "I would just say that the idea that you walk away from a negotiation that’s not satisfactory ending in your favor does show strength, in my opinion," he said. "And one of the things that we do when we get into negotiations here is we think there always has to be an agreement, and that weakens your ability to be an effective negotiator."

The timing of Session's endorsement is ironic, if not ill-fated. It comes as Trump is under scrutiny once again, this time for comments to CNN where he did not outright condemn or disavow former KKK grand master David Duke’s endorsement of him. Trump later tweeted a video from a press conference on Friday in Ft. Worth, where he told reporters that he did disavow Duke and the endorsement.

For his part, Sessions has his own history of speaking positively about the KKK. While under consideration for a Reagan appointed Federal district judgeship in Alabama, Sessions was blocked by the senate due to concerns over racially insensitive statements. Though he denied the statements and any racial prejudice, Sessions did apologize for once saying members of the Ku Klux Klan "were OK until I found out they smoked pot."

The backing comes as Alabama heads to the polls as one of 11 so-called "Super Tuesday" states in the Republican primary. Trump has been leading polls in the state.