He quickly turned to a safer line of attack, recalling that Mr. Rubio had been a leader in the push to provide a pathway to citizenship for illegal immigrants before he abandoned that legislative effort. “If you believe in something, you have to be true to yourself,” said Mr. Trump, calling Mr. Rubio “really weak on immigration.”

He added: “All of a sudden, he went down in the polls and he starts changing his tone.”…

Asked if he thought Mr. Rubio was a “threat,” Mr. Trump said he did not.

“I think he’s a baby,” he said.