Brennan, who has accused Trump of "treason," discussed in an interview with Monday with CNN's Chris Cuomo the president's decision to fire Defense Secretary Mark Esper.
"It's clear that Mark Esper was removed as secretary of defense because he rebuffed Donald Trump's efforts to politicize the U.S. military," Brennan said. "And I think it's quite apparent from reporting that Mark Esper has stood up to Donald Trump repeatedly. Who knows what else he has refused to do?"
He said Trump could do "a lot of damage" before leaving office, and he might carry out "vendettas."
Brennan repeatedly expressed concern about what "information" Trump could release, urging officials to take immediate action.
"If Vice President Pence and the Cabinet had an ounce of fortitude and spine and patriotism, I think they would seriously consider invoking the 25th Amendment and pushing Donald Trump out, because he is just very unpredictable now," Brennan said.