Donald Trump, left, and former Florida governor Jeb Bush talk over each other during the Republican presidential debate on Sept. 16 in Simi Valley, Calif. (Mark J. Terrill/AP)

If he had been president, Donald Trump said Sunday, his immigration policies would have prevented the terrorist attacks of Sept. 11, 2001. He also said fellow GOP presidential candidate Jeb Bush should stop defending his brother over the Sept. 11 attacks, as the two rivals took their feud to the airwaves on Sunday.

“I am extremely, extremely tough on illegal immigration. I’m extremely tough on people coming into this country. I believe that if I were running things, I doubt those people would have been in the country,” Trump said on “Fox News Sunday.”

“With that being said, I’m not blaming George Bush. But I don’t want Jeb Bush saying, ‘My brother kept us safe,’ because Sept. 11 was one of the worst days in the history of this country.”

Bush fired back, saying the real estate mogul’s comments about the Sept. 11 attacks show that he is not serious about being commander in chief — the central theme of his new Web ad attacking Trump.

In an interview on CNN’s “State of the Union,” Bush likened Trump’s approach foreign policy to hosting a reality TV show or playing a board game: “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.” ISIS is an alternative acronym for the Islamic State militant group.

Bush and Trump have been feuding publicly over this issue since Friday, when Trump noted in a Bloomberg News interview that “the World Trade Center came down during [George W. Bush’s] reign," appearing to suggest that the former president shared the blame for the Sept. 11 attacks.

Bush shot back on Twitter, calling Trump’s comments “pathetic” and repeating his line from the most recent GOP debate that his brother “kept us safe.” Trump said Saturday in an interview with The Washington Post: “You always have to look to the person at the top. Do I blame George Bush? I only say that he was the president at the time, and you know, you could say the buck stops here.”

On Sunday, Bush continued to defend his brother: “Look, my brother responded to a crisis, and he did it as you would hope a president would do. He united the country, he organized our country, and he kept us safe. And there's no denying that. The great majority of Americans believe that.”

He brushed off host Jake Tapper’s question about whether his loyalty may be blinding him to the mistakes made during his brother's presidency.

“No. I mean, so next week, Mr. Trump is probably going to say that FDR was around when Japan attacked Pearl Harbor. It's what you do after that matters. And that's the sign of leadership,” Bush said.