Donald Trump finally made some bold and provocative claims that were largely true, and the Republican Party finally closed ranks to attack him.

Saying Mexican immigrants are rapists didn't do it. Calling for a return of torture didn't do it. Calling for a ban on Muslim immigration didn't do it. Raising questions about Barack Obama's status as an American citizen didn't do it. Pretending that thousands of Muslims in New Jersey cheered 9/11 didn't do it.

So what did? Trump said that invading Iraq was a disaster, that the country was misled into invading Iraq by the Bush administration, and that the claim that Bush kept the country safe from terrorism is ridiculous because 9/11 happened on his watch.

It was a bizarre and telling moment, in which the battered forces of the Republican establishment finally picked themselves up off the floor specifically in order to defend some of its least defensible conduct of the 21st century.

Trump lit into George W. Bush on national security

"They lied," Trump said. "They said there were weapons of mass destruction and there were none. And they knew there were none. There were no weapons of mass destruction."

"While Donald Trump was building a reality TV show," Jeb Bush retorted, "my brother was building a security apparatus to keep us safe. And I'm proud of what he did."

Then Trump cut in with his uppercut: "The World Trade Center came down during your brother's reign. Remember that?"

A chorus of boos echoed forth from the crowd packed with establishment Republicans by the state party. Even better for Bush, Marco Rubio — in most respects his most deadly rival in the primary — stepped in to back him up.

"I just want to say, at least on behalf of me and my family, I thank God all the time it was George W. Bush in the White House on 9/11, and not Al Gore," he said. According to Rubio, the president to blame for 9/11 was not the president who was in office on 9/11; it was the guy who left office nine months earlier. "The World Trade Center came down because Bill Clinton didn’t kill Osama bin Laden when he had the chance to kill him."

The audience loved this, and were mightily displeased when Trump observed: "George Bush had the chance also, and he didn't listen to the advice of his CIA."

This is how the campaign was supposed to work

I won't even hazard a guess as to whether this double-sided exchange helped or hurt Trump. Watching it on television, you'd think Republicans there hated everything he had to say. But the reality is that the in-studio audience was hand-picked by the state party and seemingly stuffed with Bush supporters.

But if it did go badly for Trump, what's fascinating is that it went badly in exactly the kind of way you would have expected Trump's campaign to go south months ago.

He went way outside the boundaries of the kinds of things Republican Party politicians normally say, and in response Republican Party politicians (and their backers in the state party) piled on to diss him. A political party, after all, is a coalition of like-minded people. When you step outside their zone of comfort and say things they wouldn't say, they team up to crush you.

It was primary politics as it was supposed to be. And it made for a striking contrast with previous debates that had consisted largely of the establishment-friendly candidates bashing each other on the theory that whoever came out of the "establishment lane" would then face down Trump one on one at some later date. Chris Christie's murder-suicide attack on Rubio's repetition of talking points was the highest-profile example of this establishment fratricide, but in truth it's dominated the entire campaign, leaving Republicans with not much more than wishful thinking as their anti-Trump plan.

Trump was basically right about Iraq and 9/11

The strange thing is that after months of watching Trump say things that are racist, absurd, patently false, or all three at once, the Republican Party establishment decided to stomp on him for saying things that are basically true.

Most obviously, George W. Bush clearly was in office on 9/11. Repeated invocations of the notion that he "kept us safe" have managed to make this a controversial claim, but I promise you that it is true. He was inaugurated in January, and was serving as president on the morning of 9/11 when the terrorist attack momentarily interrupted his reading of My Pet Goat. Bush received repeated warnings about al-Qaeda plots against the United States, and his administration was given a plan to tackle al-Qaeda and the Taliban that it rejected as a holdover from the Clinton administration and a distraction from bigger problems.

Trump's claim that the Bush administration positively knew there were no WMDs in Iraq is more dubious, but it's unquestionably true that the sort of WMD programs the White House said existed weren't found and that the administration's public presentations of intelligence findings were highly skewed and selective.

By Trump standards — this is a man, after all, who claims he can make Mexico pay for the construction of thousands of miles of border wall — these arguments are tame. Indeed, almost banal. For months now, Republicans have wondered how Trump could be winning by claiming up was down. But this was exactly how they won in their mid-aughts heyday — slamming decorated war hero John Kerry for cowardice, claiming to have kept the country safe while presiding over the worst terrorist attack in American history, and responding by invading an unrelated country in order to dismantle a nuclear weapons program that didn't exist.

When Trump contradicted the Republican Party's most cherished form of up-is-downism, the party establishment finally got its groove back and handed him arguably his worst evening of the entire campaign. But they also proved to everyone else that pointing out that Bush was in office on 9/11 is a red line for the GOP establishment in a way that concocting a story about Jersey City Muslims celebrating the attacks wasn't.