President Donald Trump promised to campaign for Roy Moore should he be victorious over Luther Strange -- the candidate whom Trump endorsed -- in Tuesday's Republican primary.

Trump was in Huntsville on Friday to rally behind Strange -- and briefly went off message by bringing up Moore. But Trump later turned his promise into an opportunity to slam Moore.

"I told Luther, and I have to say this, if his opponent wins, I'm going to be here campaigning like hell for him," Trump said. "But I have to say this, and just understand this .. Luther will definitely win."

Trump said both Moore and Strange are "good men," but he turned his offer into campaigning for Moore into a slight against the former chief justice of the Alabama Supreme Court.

"Roy has a very good chance of not winning in the general election," the president said. "Luther's going to win easily and Roy's going to have a hard time winning, but I will be backing him if he wins."