U.S. Republican presidential candidate Donald Trump introduces Alabama Senator Jeff Sessions Mobile during his rally at Ladd-Peebles Stadium on Aug. 21, 2015 in Mobile, Ala. 
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Alabama’s Jeff Sessions becomes first senator to endorse Trump

Updated

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!”

Sessions’ endorsement comes on the heels of another high-profile endorsement 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.

“I’m becoming mainstream. All these people are now endorsing me,” Trump told the sprawling Alabama crowd. “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 in crafting the Trump’s immigration policy plan.

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

The endorsement comes as Alabama heads to the polls on Tuesday, one of the states participating in so-called “Super Tuesday.” Trump is leading in polls in the state.

This story first appeared on NBCNews.com.

Donald Trump and Jeff Sessions

Alabama's Jeff Sessions becomes first senator to endorse Trump

Updated