"I told Luther, I have to say this, if his opponent wins, I'm going to be here campaigning like hell for him," Trump told a crowd in Huntsville, Ala.

Trump traveled to Alabama to stump for Strange days before he faces off against Roy Moore, a former Alabama Supreme Court chief justice, in Tuesday's runoff.

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Moore has led in polls while campaigning with several top Trump allies, while Strange has boasted the support of Trump and the GOP establishment.

Trump insisted Friday night that Strange had a better chance of winning the primary and going onto win the general election.

"I have to say this, and you understand this, and just look at the polls. Luther will definitely win," he said. "Roy [Moore] has a very good chance of not winning in the general election."

The president said if Strange does not win, he would be blamed for the loss.

"I might have made a mistake. And I'll be honest, I might have made a mistake, because, you know, here's a story, if Luther doesn't win, there's a very short period of time. They're going to say, 'Donald Trump, the president of the United States, was unable to pull his candidate across the line,' " he said.

The endorsement has driven a wedge between Trump and some key allies in and out of his administration, who have backed Moore.

Moore has also earned the endorsements of Trump's former chief strategist Stephen Bannon and former Alaska Gov. Sarah Palin (R).

Trump insisted Friday that Strange would "end business as usual" in Washington and argued that Strange was not close to the Republican establishment in D.C.

"Luther Strange is determined to drain that swamp," Trump said to applause.

"He doesn't know those people. He never met them, he doesn't know them," he continued. "He shares our agenda. Remember, he's going to win the race."

Updated: 11 p.m.