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Madison, Alabama CNN —  

Sen. Jeff Sessions on Sunday became the first sitting senator to endorse Donald Trump, declaring his support for the Republican front-runner during a rally at a football stadium here.

The endorsement from the Alabama Republican known for his opposition to illegal immigration will solidify Trump’s attempts to position himself as the Republican presidential candidate with the toughest stance on the issue.

The endorsement comes two days before the crucial Super Tuesday primary when Alabama and a dozen other states, mostly in the South, vote.

“At this time in American history we need to make America great again,” Sessions told the raucous crowd as he donned one of Trump’s signature caps emblazoned with that campaign slogan. “I am pleased to endorse Donald Trump for president of the United States.”

“A movement is afoot that must not fade away,” Sessions added.

Cuing up Sessions’ endorsement on stage Sunday before a crowd of thousands, Trump referred to the Alabama senator as “really the expert as far as I’m concerned on borders.”

“We want strong borders. We want the wall,” Trump said.

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Sessions’ endorsement has been months in the making. Speculation of a potential endorsement has swirled since Sessions appeared with Trump at his massive rally in Mobile, Alabama last summer and donned a “Make America Great Again” cap.

The Alabama senator then advised Trump as he crafted and ultimately published proposals to reform the U.S. immigration system. And last month Sessions’ communications director Stephen Miller joined Trump’s team as a policy adviser.

The Sessions endorsement allowed the Trump campaign to again set its own agenda. It came after Trump’s campaign spent much of Sunday explaining comments on CNN that he didn’t know enough to disavow former Ku Klux Klan grand wizard David Duke. Trump later tried to clean up his comments on Twitter amid an outpouring of criticism from his Republican presidential rivals. Sen. Marco Rubio slammed the remarks, saying they make Trump “unelectable.”

Trump on Monday tried to clean up the mess, saying he disavowed white supremacists and blamed the incident on a bad earpiece.

Sessions said Trump needs to make clear where he stands.

“We believe in equality and fair treatment and that’s the moral principle we adhere to as a nation and I hope he makes that clear,” Sessions said on Monday on “The Matt Murphy Show.” “He’s disavowed this before, and you get asked these questions, I don’t know what happened. But I will say this, he needs to make that clear.”

Immigration has been at the center of Trump’s presidential platform since the billionaire launched his presidential bid amid controversy, after calling undocumented immigrants coming into the U.S. from Mexico “criminals” and “rapists.”

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Trump has since pledged to build a wall on the border between the U.S. and Mexico and promised to force the Mexican government to pick up the cost.

Sessions’ is the second major endorsement Trump has received in a matter of days. New Jersey Gov. Chris Christie, a former rival of Trump’s for the Republican nomination, endorsed Trump on Friday.

Sessions, who has been in Congress for nearly two decades, said Sunday that he believes Trump will be best positioned to secure the border and stop illegal immigration, which he said “politicians have promised for 30 years to fix.”

“Have they done it?” Sessions asked to a resounding “No!” from the crowd. “Donald Trump will do it. He will.”

Sessions also touted Trump’s outsider status, noting that Trump – who largely self-funds his campaign – is not backed by special interest groups.

Sessions added that Trump’s campaign “has the potential to have the American people’s voices heard for a change.”

“The bosses are you! People in Washington are public servants. They serve you and we haven’t had enough of that,” Sessions said, before ceding the stage to Trump.

CNN’s Tal Kopan contributed to this report.