Graham said that Trump's knowledge of foreign policy "scares the hell out of me" and described his proposals as "gibberish."
"Donald Trump, I think, is the most unprepared person I’ve ever met to be commander in chief," he said, as quoted by the Times.
The South Carolina senator said that both Trump and Cruz would secure a win for Democratic presidential frontrunner Hillary Clinton.
"Dishonest—which is Hillary Clinton in the eyes of the American people—beats crazy," he said.
"If you take those two off the table, dishonest loses to normal, so let’s just pick somebody out of the phone book if we have to," Graham continued. "We can win this election unless we lose it."