"Look, my brother responded to a crisis, and he did it as you would hope a president would do," Bush said of his older brother during an interview on CNN's "State of the Union."
"He united the country," Bush continued. "He organized our country, and he kept us safe. And there's no denying that. The great majority of Americans believe that."
Bush was responding to criticism about his older brother's foreign policy record from Republican presidential front-runner Donald TrumpDonald TrumpHeadaches intensify for Democrats in Florida Stormy Daniels set to testify against former lawyer Avenatti in fraud trial Cheney challenger wins Wyoming Republican activists' straw poll MORE.
He said Trump is trying to score political points by talking about the Sept. 11 attacks that were a touchstone in his older brother's presidency.
"I don't know why he keeps bringing this up," Bush said of Trump's 9/11 criticism. "It does show that he's not a serious person as it relates to being commander in chief and being the architect of a foreign policy.
"Across the spectrum of foreign policy, Mr. Trump talks about things as though he's still on 'The Apprentice,' " Bush continued. "I mean, literally, talking about Syria, saying ISIS should take out Assad, then Russia should take out ISIS, as though it was some kind of board game, and not a serious approach.
"This is just another example of the lack of seriousness," Bush said. "And this is a serious time. We're under grave threats again, and I think we need a president with a steady hand."
Trump took to Twitter on Sunday morning, asking if the former president had "bad info" for attacking Iraq.
Jeb, why did your brother attack and destabalize the Middle East by attacking Iraq when there were no weapons of mass destruction? Bad info?— Donald J. Trump (@realDonaldTrump) October 18, 2015
— This report was updated at 10:06 a.m.