Ted Cruz just pulled off what has seemed impossible: He made Donald Trump concede a point.

At Thursday night's Republican debate, Cruz took on Trump's charge that he's not American enough for the presidency. Trump has recently started saying Cruz's Canadian birth is a constitutional liability. When asked about the attacks Thursday night, Cruz's second response was to explain why a child of a US citizen is eligible to be president — but his first response was to boast that Trump was feeling threatened in the polls.

Cruz is a champion debater, and he had to have known this question was coming. So the sharpness of his answer doesn't come as a surprise. But Trump has shown an uncanny ability (even for a presidential candidate) to avoid admitting he was wrong. He's denied the existence of statements that are on his own website just to avoid admitting he contradicted himself. Cruz got under his skin.

Trump sputtered. But he ended up admitting to moderator Neil Cavuto that Cruz was right.

Donald Trump is hitting Cruz with an electability troll

The reason this issue has come up is that Ted Cruz was born in Canada, to a US citizen mother and a noncitizen father. He's definitely a citizen. But the Constitution requires that presidents be not just citizens, but "natural-born citizens" — and it doesn't define what "natural-born" means.

Most constitutional lawyers think that if the question were to come up in court, a judge would rule that Cruz does count as a natural-born citizen. But because the case hasn't come up in court yet, the question is not yet legally settled.

That's why Harvard law professor Laurence Tribe — whom Trump has been citing on the campaign trail, and whom he cited again during the debate — has started saying that Cruz's citizenship could be a problem. Tribe personally believes Cruz should be considered a citizen, but he says that if Cruz were consistent about his own constitutional philosophy, Cruz would believe he was ineligible. More importantly, Tribe says, the unsettled question would throw a "cloud" over Cruz's candidacy.

Trump is saying exactly the same thing. In the past, he has said directly that he doesn't think Cruz is a natural-born citizen. But in this campaign — and in the debate — he said something slightly different: that it doesn't matter what he thinks, because it would be a bad idea for the party to nominate someone who might have to fight a court case on his way to the White House.

Let's call this what it is: an electability argument. Just like candidates who claim that their opponents are too extreme to win over swing voters in the general election, Trump is claiming that Cruz's Canadian citizenship would become a problem for the party if he wins the nomination.

Cruz's counterargument worked because his conservative cred is more secure than Trump's

The problem with the electability argument, in this case, is that it's easy to rebut when the other party is attacking you. Why would they be going after me if they weren't worried I'd beat them? And that's what Cruz did in the debate.

One of the strange things about the Trump candidacy has been that his populist policy platform has coexisted with some of the gaudy East Coast elitism of his previous life as a New York real estate mogul. Most Republican candidates wouldn't trot out a Harvard Law professor to buttress their points — and certainly wouldn't cite one with ties to the Democratic Party. But Trump did.

As a result, Cruz had an opening to remind the audience that Trump himself was a Democrat for the early years of his career; that Hillary Clinton attended his wedding; that he used to be a pro-choice, pro-government-health-care liberal. And indeed, that's a theme he ended up hitting more directly later in the debate, when he accused Trump of having "New York values."

But Cruz didn't have to say any of those things to defend his own citizenship. He only had to point out that Trump was willing to side with Harvard Law professor and former Al Gore adviser Laurence Tribe, because Trump was so scared Cruz would win.