COLORADO SPRINGS, Colo. -- Donald Trump launched a series of fierce attacks against former Defense Secretary Robert Gates on Saturday.

"He's a nasty guy. Probably has a problem that we don't know about," Trump said at a rally in an airplane hangar here. Trump attacked Gates on Twitter earlier in the day, calling him "dopey."

Trump told his supporters he has never met Gates but has concluded that, "he's a mess."

Gates expressed worries about the preparedness of both Trump and Hillary Clinton to be commander-in-chief in a Friday Wall Street Journal op-ed. He wrote that on national security,"I believe Mr. Trump is beyond repair."

Gates added: "He is stubbornly uninformed about the world and how to lead our country and government, and temperamentally unsuited to lead our men and women in uniform. He is unqualified and unfit to be commander-in-chief."

The former Defense secretary under George W. Bush and Barack Obama wrote that Clinton "has time before the election to address forthrightly her trustworthiness" and "to reassure people about her judgment." But he said he does not know whether she will.

"I am so much better at what he's doing than he is," Trump said of Gates.