MADISON, Alabama — 2016 GOP presidential frontrunner billionaire Donald Trump picked up the most significant endorsement any presidential candidate in the GOP can get here on Sunday: Sen. Jeff Sessions (R-AL).

Sessions, the intellectual leader of the future of the conservative movement, has provided the brainpower behind the populist nationalist revolt against political elites that’s been emerging since at least 2013.

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At a warm and windy rally here with thousands present in a packed football stadium just outside Huntsville, Sessions appeared on stage with Trump to back him for president. Sessions’ endorsement provides Trump with even more legitimacy as Trump’s two remaining serious opponents—Sens. Marco Rubio (R-FL) and Ted Cruz (R-TX)—attempt to undermine him in a desperate bid by the donor class to regain control of the party from populists revolting in elections around the country. Sessions backing Trump is a significant blow to both Rubio and Cruz, as now the powerful Alabamian will be putting his entire operation all in behind Trump.

Sessions’ backing is the latest and highest-profile in a string of significant endorsements for Trump’s rebel campaign. New Jersey Gov. Chris Christie, after dropping out of the race himself despite landing significant blows on Rubio in the final debate before New Hampshire’s primaries, backed Trump this week, as did Maine Gov. Paul LePage and former Arizona Gov. Jan Brewer, as well as Reps. Duncan Hunter (R-CA) and Chris Collins (R-NY).

But all those endorsements pale in comparison to Sessions’ backing of Trump, as the Alabamian senator is universally respected—and feared—inside the beltway in Washington. Sessions is known for his tough views on immigration and trade and has repeatedly aimed to push the Republican Party in a more populist, nationalist direction. Sessions has helped frame the movement’s views on both issues—and more, like courts and judicial nominations, law and order and police matters, and on budgetary issues and the fiscal well-being of America—while maintaining a formidable political and policy operation that his allies view as invaluable and his enemies dread.

Sessions is the most highly sought-after endorsement because of his respect party-wide, and he’s been cited by Cruz especially frequently on the campaign trail. He has appeared with Cruz at events in Alabama, and with Trump in Mobile five hours south of here earlier in the cycle. He has also helped many of the GOP candidates, including Cruz and Trump, craft their policy positions and has been—alongside his numerous allies on Capitol Hill in the House and Senate—a driving force in framing the populist nationalist future of the Republican Party.

“Donald, welcome to my hometown, Mobile, Alabama,” Sessions said back in September when he appeared with Trump in Mobile—while wearing a “Make America Great Again” trucker hat that Trump has popularized—not an official endorsement then. “The American people, these people, want somebody in the Presidency who stands up for them, defends their interest, and the laws and traditions of this country. We welcome you here. Thank you for the work that you’ve put into the immigration issue. I’m really impressed with your plan. I know it will make a difference. And this crowd shows a lot of people agree with that. Congratulations. Welcome, and God bless.”

Sessions’ old communications director Stephen Miller joined Trump’s campaign earlier in the cycle as a senior policy adviser, and the two populists have been close before. But now it’s official with Sessions’ public endorsement of Trump.

Now that Sessions has officially backed Trump for president, his enormous respect among Republican voters nationwide—and especially in Super Tuesday states like here in Alabama—and among Republicans party-wide in Washington is sure to give Trump a game-change-level boost heading into Super Tuesday and future states.

If Trump wins big on Super Tuesday—he’s polling ahead of his rivals in almost every Super Tuesday state, except Cruz’s Texas—it sets him up for a significant battle to take down Rubio in Florida. Currently, Trump is polling well ahead of Rubio in the Florida senator’s home state—an embarrassment for the young first-term senator. A win by Trump over Rubio in Florida would almost certainly end the Rubio presidential campaign, as the young senator and his campaign have promised publicly they will win Florida.