WATCH: CNN Grills Jeb Bush Point Blank: Did Your Brother Fail To Stop 9/11?

Playing devil's advocate, CNN's Jake Tapper spent part of his show Sunday morning grilling GOP presidential candidate Jeb Bush about his brother George W.'s failure to prevent the 9/11 terror attacks that happened on his watch as president. The episode followed a similar call-out by GOP front-runner Donald Trump during the second Republican debate earlier this month and repeated needling by Trump since then for Bush's supporting his brother's performance on national security in the months preceding the al Qaeda attacks.

"Look, my brother responded to a crisis as you would hope a president would do: He united our country, he organized our country, and he kept us safe," Bush said to Tapper on "The Lead." "And there's no denying that."

Critics other than Trump have pointed out the infamous memo received by the Bush adminstration on Aug. 6, 2001, labeled “Bin Laden Determined to Strike in U.S.” and several classified reports preceeding it throughout June and July.

Tapper asked whether the former Florida governor's loyalty to his brother was a liability obscuring his judgment.

"It's what he did afterward that mattered," Bush went on. "And I'm proud of him and so are a lot of other people. You don't need to have your last name be Bush to understand that."

Bush has stumbled over the question of his brother's legacy throughout the campaign, particularly in May when asked by Fox News Channel's Megyn Kelly whether he would have invaded Iraq in 2003, as W. did, knowing what he knows now.

"I would have," Bush said. Jaws dropped from left and right -- as his rivals capitalized on his misstep supplying their own answers -- and Bush spent the next week slowly walking back the statement until he gently relented and conceded that "knowing what we know now," that "I would not have gone into Iraq."

On Sunday, Tapper took the questioning on the matter of blame one step further.

"Obviously al Qaeda was responsible for the terrorist attack of 9/11. But how do you respond to critics who ask, if your brother and his administration bear no responsibility at all, how do you make the jump that President Obama and Secretary Clinton are responsible for what happened at Benghazi?"

"There were calls for security; it looks like they didn't get it," Bush answered. "Was there a chance that these four American lives could have been saved? That's what the investigation's about. It's not a political issue."