"If Donald Trump is the nominee, that’s the end of the Republican Party," Graham said.
Graham was defending his dismal numbers in his home state of South Carolina, where the latest poll showed him earning 4 percent of likely Republican primary voters to Trump's 30 percent.
Graham continued his attack, criticizing Trump's lack of military and foreign policy experience.
"He's shallow. He’s ill-prepared to be commander in chief," the retired Air Force colonel told CNN. "He's a complete idiot when it comes to Mideast policy."
Graham said "common sense" will win out, but until then, "Our soldiers deserve better."
Graham said that Trump's views on immigrants and women do not reflect the modern Republican party.