© J. Scott Applewhite / APSen. John McCain
"I don't agree with the way he's conducting his presidency, obviously," McCain said during an interview with Esquire.
"He's an individual that unfortunately is not anchored by a set of principles. I think he's a person who takes advantage of situations."
McCain said Trump was "successful" as a builder and an entrepreneur.
"But I don't think he has the fundamental underpinnings of principles and beliefs," he said.
He added: "I don't think there was any doubt about his views toward me. But I'm a loyal Republican."
McCain has criticized Trump in the past. Last month, McCain during an event blasted the "half-baked, spurious nationalism" in the United States.
In response to McCain's comments, Trump warned: "At some point, I fight back and it won't be pretty."
McCain later shot back, saying: "I've faced far greater challenges than this."
Trump also attacked the Arizona Republican after he voted against a GOP bill to repeal and replace ObamaCare.
McCain's dramatic vote killed the bill and left ObamaCare the law of the land.
Trump later said McCain "let down" his party and the people of Arizona by opposing the measure.
Comment: Whatever, John: