ally Newt Gingrich said Wednesday that the GOP presidential nominee has been "self-destructive" and needs to "grow into the size of the job."

"I think some of what Trump has done is very self-destructive," Gingrich added.

Trump on Tuesday snubbed Ryan and McCain, who have both stood by their endorsements of him while criticizing his feuding with the parents of a slain American Muslim soldier.

Trump saying he wasn't ready to endorse Ryan and criticizing McCain's work on behalf of veterans cast doubt on GOP unity amid already swirling criticism over Trump's feuding with the soldier's family.

Gingrich likened Trump's candidacy to being on "The Apprentice," but said in this situation, "He doesn't get to say, 'You're fired.' The American people get to say you're fired."

"I think he has the potential to win the election, I think he has to potential to be a historic president, but to do that he's got to grow into the size of the job," Gingrich said.