The wildest exchange of the Thursday night Fox News Republican debate was a free-for-all over immigration — Marco Rubio versus Jeb Bush versus Megyn Kelly versus Ted Cruz versus Rand Paul.

No one looked particularly good. But Cruz is the only one who lost.

It might not be the exact moment that sinks Cruz's candidacy. Donald Trump appears to deserve the credit he's taking for that. But it was definitely the moment when the bitter animosity people feel toward Cruz — his fellow candidates, his fellow senators, and even, apparently, Fox News — broke out in the open.

At the heart of the attacks on Cruz is that behind his claims of conservative purity, he is, in fact, no better than the establishment candidates on the stage. He may not flip-flop as often as they do, but he's just as interested in doing what it takes to get ahead.

That attack, especially when applied to such a touchy issue as immigration, robs him of his key appeal to the conservative base. And it leaves his chief nemesis — Donald Trump — not only with an uncontested claim to immigration hard-liner cred, but as the only Republican candidate who actually speaks from the heart.

Who accused whom of what, explained

I am an immigration beat reporter, I have watched every Republican debate this cycle, and even I had trouble keeping track of the various claims candidates and moderators were throwing at each other.

It helps to think of the fight as a series of accusations — some of which have come up in previous debates. Here's a chronological breakdown and a quick assessment.

1) Marco Rubio promised not to support "amnesty" when he was running for Senate. Then he did.

The accusation: Megyn Kelly kicked off the immigration segment by producing some old clips from Rubio's 2010 campaign for US Senate, during which he said that an "earned path to citizenship" for unauthorized immigrants in the US was a euphemism for "amnesty" — and he opposed "amnesty." This set up a different spin on the question that Rubio's been asked before: Wasn't he supporting amnesty when he backed the Gang of Eight immigration reform bill in 2013?

Is it true? Yes.

I'm sympathetic to Rubio here. Politicians tend to use "amnesty" to describe whatever immigration policy they don't like — and plenty of politicians say they oppose amnesty but support comprehensive immigration reform, because they don't think there's overlap between the two. So asking a politician, "Haven't you flip-flopped when you used to say you were anti-amnesty?" is a gotcha question nine times out of 10.

This is the 10th time. Using "I oppose amnesty" as a dodge against specifics only works if you don't define amnesty. And the clip showed Rubio doing just that — saying that "earned citizenship" and amnesty were the same thing. That means that by the definition of 2010 Rubio, 2013 Rubio was definitely an amnesty supporter.

Did he defend himself? No. Rubio usually has at least a semi-decent response to questions like this. But he had a poor debate performance Thursday, and this was part of it. He stammered an answer about ISIS that was wholly unresponsive to the charges against him.

2) Marco Rubio ran away from his own bill when the going got tough

The accusation: Jeb Bush appeared to jump to Rubio's aid about the 2013 immigration bill — "I supported him because I think people, when you're elected, you need to do things, and he led the charge" — only to knife him between the ribs: "Then he cut and run because it wasn't popular amongst conservatives, I guess."

Is it true? Yes.

Rubio was deliberately picking a fight within his own party when he backed the 2013 bill. He even did a media tour of conservative talk radio to defend the bill, because he understood how the base would feel about it.

Rubio's plan backfired: He got all of the downside and none of the upside. He bailed on his own bill shortly after it passed the Senate — helping cement the idea that it was politically toxic for House Republicans. But it was too late for him to save his own reputation.

Did he defend himself? No. He's tried to get around this charge in the primary by attributing his change of mind to events: namely, the border crisis of 2014 and the rise of ISIS. That line hasn't always been persuasive. Tonight, after his total failure to respond to Kelly's question, it definitely was not.

3) Jeb Bush flip-flopped on immigration in his book

The accusation: Marco Rubio counterattacking against Bush: " It's interesting Jeb mentions the book; that's the book where you changed your position on immigration, because you used to support a path to citizenship."

Is it fair? Yes.

Bush's 2013 book, Immigration Wars, was an attempt to extend an olive branch to the hard-liners in his party, by recommending that unauthorized immigrants living in the US be allowed to get legal status but not citizenship. Unfortunately for Bush, the timing was epically bad; the book came out right as the Gang of Eight had introduced principles for an immigration bill involving citizenship.

The flip-flop didn't attract any praise from the base. It did, however, attract the attacks of immigration reformers, who felt Bush was abandoning both his own principles and the reformers within his own party. Rubio predicted that Bush would "come around on citizenship." He was right; Bush flip-flopped again shortly after his book was released.

Did he defend himself? No. This exchange ended with Bush and Rubio sounding like preschoolers in the back seat, sneering, "You flip-flopped too!" at each other. It looked terrible and bizarre for both of them — cementing the criticism that Megyn Kelly had raised to begin with, that Rubio (and now Bush) was not to be trusted on immigration.

4) Ted Cruz would have supported the Gang of Eight immigration bill if it had only legalized unauthorized immigrants without giving them citizenship

The accusation: Here's where it gets interesting.

Kelly's question to Cruz was similar to her question to Rubio. She introduced an archival clip, from the 2013 debate over the Senate bill, in which Cruz appeared to say he supported allowing unauthorized immigrants to get legal status as long as they couldn't become citizens. Then she asked: "Was that all an act? It was pretty convincing."

Is it true? Maybe.

This goes back to an amendment Cruz offered to the Senate bill in 2013, when it was being considered by the Senate Judiciary Committee. The backstory is complicated, but the short version is that Cruz and his defenders say it was just a tactic to demonstrate that Rubio and company weren't willing to compromise, while his detractors say it was a serious proposal to legalize unauthorized immigrants.

Cruz pointed out Thursday that the amendment was only a handful of words — not exactly sophisticated enough to be a serious proposal. For what it's worth, that was my reaction when he filed it as well: If Cruz really wanted the bill to legalize unauthorized immigrants without granting them citizenship, he didn't put a ton of time into writing down how that would work.

But no one will ever know what was in Ted Cruz's heart in 2013. The point is that his amendment was strategically vague — focusing on his common ground with others in his party, to draw attention away from his disagreements with them.

Did he defend himself? Not exactly.

Cruz's "poison pill" defense didn't make him sound like a flip-flopper, necessarily. But it made him sound like a politician — someone who was putting forward a policy for his own ends, rather than because he felt it was good for America.

That goes against the heart of Cruz's appeal: that he is a dyed-in-the-wool conservative who is inflexibly committed to conservative principles. It also seems to validate the reason that Cruz's fellow senators hate him so much: He's perfectly willing to use conservative principles to bash his colleagues, but ultimately the only person he's interested in serving is Ted Cruz.

That's why the peak of the immigration exchange was actually from a fourth candidate: Rand Paul. Paul all but accused Cruz of being a self-righteous hypocrite — something it's been known for years that many of his fellow Republicans thought of him but that no one has said out loud on primetime television:

What is particularly insulting, though, he is the king of saying, oh, you're for amnesty, everybody's for amnesty except for Ted Cruz, but it's a falseness. And that's an authenticity problem that everybody he knows is not as perfect as him because we're all for amnesty.

5) Ted Cruz helped develop George W. Bush's comprehensive immigration reform policies

The accusation: Sen. Marco Rubio gleefully joined Sen. Rand Paul in beating up on the stuck-up self-righteousness of Sen. Ted Cruz: "This is the lie that Ted's campaign is built on and Rand touched upon it that he's the most conservative guy and everyone else is, you know, everyone else is a RINO."

But instead of going back to 2013, Rubio added another accusation: that Cruz had worked with George W. Bush on Bush's immigration policy, which also happened to include a path to citizenship for unauthorized immigrants.

Is it true? Not really.

Cruz was already out of the Bush administration during Bush's second-term battles on immigration reform. What Rubio is probably referring to is a 1999 memo that Cruz wrote for Bush's presidential campaign, which included (according to the New York Times) this passage:

"Not at this time," Mr. Cruz suggested that Mr. Bush, then the Texas governor, say if asked whether he supported "amnesty for illegal aliens already here." "I think we need to consider all options when trying to resolve our immigration problem and what to do with the millions of illegal immigrants already living here," Mr. Cruz wrote in a five-page memo, offering Mr. Bush some flexibility on what has long been a thorny issue for Republicans. "But, right now, I don’t think amnesty is the answer."

Did he defend himself? No. He didn't even try — Cruz just returned to 2013 to draw a contrast between himself and Rubio. If he had tried, he would have revealed that the Paul/Rubio critique of him wasn't far off.

What's really going on here

Ted Cruz has been extremely careful about what he's said on immigration. He's never flip-flopped. But he's definitely emphasized different things for different audiences.

That just makes him a politician. But since Ted Cruz doesn't have any sympathy for people who act like politicians — since he is, in fact, the bane of his party because he scourges them for acting like politicians — it's easy to see why it angers his colleagues so much.

Bush and Rubio weren't going to revive their flagging campaigns by attacking each other. There have already been tens of millions of dollars spent on that fight, with the predictable result that neither Bush nor Rubio is doing terribly well while Donald Trump cruises to victory.

But when Megyn Kelly turned to Cruz, it suddenly seemed as if everyone onstage, candidates and moderators alike, had been waiting for this moment to take him down. It was almost as if Fox News — which appeared to have it in for Cruz throughout the debate — had engineered the immigration segment as a way to ambush Cruz with the clip of his speech on the Senate floor.

If Cruz actually believes what Bush and Rubio say he believes, they agree with Cruz more than they agree with Trump. But they — and Rand Paul, and Fox News — were still willing to use that to bloody Cruz. Just like much of the party appears to be willing to nominate Trump, as much as they despise him, if that's what it takes to keep the nomination away from Cruz.

VIDEO: Rand Paul's revenge on Ted Cruz

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