Donald Trump accused George W. Bush of failing America on 9/11 — in an ugly exchange with Jeb Bush during Saturday night's Republican debate.

Trump started the fight with Bush, calling him out for his vague answer on the Iraq War last year.

Bush swung back hard at Trump, making it personal too: "While Donald Trump was building a reality TV show, my brother was building a security apparatus to keep us safe. And I'm proud of what he did."

Trump jumped in with the harshest line: "The World Trade Center came down during your brother's reign. Remember that."

Trump's argument is simple: If George W. Bush really kept America safe, as Jeb Bush claims, then why did 9/11 happen under George W. Bush's watch?

Trump's made this argument before: He told Bloomberg TV in 2015, "When you talk about George Bush, I mean, say what you want, the World Trade Center came down during his time."

But this is the first time Trump has made the argument on the debate stage.

Will it play well with Republicans, who may not appreciate Trump going after a Republican president? It's hard to say. But the crowd certainly didn't appreciate it, repeatedly booing Trump for his remarks.