"It's not just absurd, it's offensive. It's ridiculous. And I do think it's a disqualifier as commander-in-chief," Rubio said in an interview Sunday with CNN's Jake Tapper on "State of the Union."
The Florida senator was the latest Republican White House contender to hit Trump after the real estate mogul said Saturday that McCain "is not a war hero" because he was captured and held prisoner in Vietnam.
"He is a war hero because he was captured," Trump said. "I like people that weren't captured, OK?"
He walked back those comments on Twitter later, saying that "captured or not, all our soldiers are heroes!" But opponents who'd been looking for openings to criticize Trump were already piling on.
Rubio defended Republicans who were slow to criticize Trump over his controversial comments about undocumented Mexican immigrants a week ago, too, saying that it's been Trump's rise in the polls that has "required people to be more forceful on some of these offensive things he is saying."
"I spoke out against what Donald Trump said," Rubio said. "But we have to remember, this is a man who spent his whole life saying outrageous things."
Trump defended himself Sunday, issuing a statement touting his financial contributions to veterans' causes and criticizing Veterans Affairs hospitals with long waits for military veterans to receive medical care.
He also explained his attack on McCain, saying his anger is because "politicians like John McCain have totally failed" at tackling illegal immigration.
He pointed to McCain's reaction to a Trump event in the Arizona senator's home state.
"Last week in Phoenix, I held a record breaking rally with thousands of people. They are devastated by what is happening with illegal immigration. This group was totally united -- the silent majority became alive," Trump said.
"These were fabulous people and great Americans but John McCain called them 'crazies,'" Trump said. "Everyone was very insulted by his disgusting and unfair characterization. A nation without borders is not a nation at all but John McCain has not been able to figure that out."