The endorsement represents a major blow to Sen. Ted Cruz (Tex.), one of Trump's two chief rivals for the Republican nomination. Cruz has touted his strict positions on border security and deportation, leaning on his strident commitment to conservative ideology as a key rationale for his candidacy. In the run-up to the March 1 Super Tuesday primary elections, Cruz has tried to undermine Trump's conservative bona fides on immigration reform, characterizing his plan as “amnesty.”

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The announcement is the latest in a series of high-profile endorsements by prominent GOP voices including New Jersey Gov. Chris Christie and former Arizona governor Jan Brewer. Sessions’s appearance came as a surprise for those in attendance, who cheered when he took the stage.

“When I talk about immigration and when I talk about illegal immigration and all the problems with crime and everything else, I think of a great man,” Trump said in introducing Sessions. “Sweet Home Alabama” finished playing as Trump took the stage.

“That is so great. You know, he’s an incredible guy,” Trump said after Sessions spoke.

Trump has made immigration reform and border security a linchpin of his presidential run, calling for the mass deportation of 12 million undocumented immigrants living in the United States and promising to build a massive wall alongside the U.S.-Mexico border. Many of Trump’s critics have accused him of xenophobia; others have argued that his plan to build the wall — and to force Mexico to pay for it through aggressive diplomacy — is unrealistic.

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The Sessions endorsement also comes as Cruz and Sen. Marco Rubio (Fla.) — who has been locked in a bitter fight with Cruz for second place — have made an aggressive push to characterize Trump as a false conservative who has made promises he does not intend to keep.

Cruz and Sessions, one of the most conservative members of the Senate, have forged a close relationship in recent years. Cruz has regularly made reference to Sessions on the trail, emphasizing the mutual respect between the two.

Sessions appeared with Cruz in Daphne, Ala., in December and has defended the senator from Texas against accusations that he once supported a path to legal status for undocumented immigrants. Sessions has supported Cruz’s claim that an amendment he offered, which stripped the bill of a path to citizenship for undocumented immigrants but granted legal status, was intended as a "poison pill" that would kill the legislation.

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But the relationship between Trump and Sessions has slowly warmed over the past year. Starting early last year, Sessions and his advisers privately consulted with Trump on immigration policy and helped to shape the candidate’s position paper. They had one extensive call in July that was confirmed by both sides and is considered the moment that Sessions began to consider backing Trump.

“I like him,” Trump said of Sessions when asked about that call. “Tough guy. I like that. We have a similar thought process.”

By August, they were in more frequent touch — two men who shared staunch conservative views on trade and immigration. When Trump held a rally in Mobile, Ala., in August, he did so in part because it is the home town of Sessions, whom he brought onstage as thousands roared. The senator put on a “Make America Great Again” cap.

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A month later, when Trump visited Washington in September for an event, he huddled with Sessions in the senator’s hideaway office at the Capitol. Former Alaska governor Sarah Palin, who would later endorse Trump, joined them.