One of the best exchanges of the sixth Republican presidential debate came thanks to The Washington Post. When our Robert Costa and Philip Rucker asked Donald Trump about Ted Cruz's having been born in Canada, Trump replied that it could be a problem -- kicking off more than a week of debate over whether or not Cruz was a "natural-born citizen" eligible to be president under the constitution.

The topic came up during the debate after Cruz was asked to respond to the issue. The Texas senator wasted no time in engaging Trump on the topic -- even suggesting that Trump's citizenship might be questionable.

CRUZ: At the end of the day, the legal issue is quite straightforward. But i would note that the birther theories that Donald has been relying on, some of the more extreme ones insist that you must not only be born on U.S. soil, but have two parents born on U.S. soil. Under that theory, not only would I be disqualified, Marco Rubio would be disqualified, Bobby Jindal would be disqualified and, interestingly enough, Donald J. Trump would be disqualified.

TRUMP: Not me!

CRUZ: Because -- because Donald's mother was born in Scotland. She was naturalized.

TRUMP: But I was born here. Big difference.

CRUZ: On the issue of citizenship, Donald, I'm not going to use the issue of your mother's birth against you.

TRUMP: Because it wouldn't work.

Trump's right. Cruz's theory -- certainly proposed for rhetorical purposes and not sincere ones -- is not one that's actually embraced by legitimate scholars. (And to Cruz's credit, he did label the theory "one of the more extreme ones.")

As we have noted before, the United States legal code doesn't articulate what counts as "natural-born" in the constitutional sense, but it does outline very clearly who counts as a citizen at birth. Nowhere is it dictated that the parents have been born in the country.

Where'd it come from? It seems to have originated with Orly Taitz. Taitz is a well-known conspiracy theorist who was at the center of the fight over Barack Obama's citizenship -- arguably even more deeply than Trump. A dentist and lawyer, her blog became a locus of birther arguments during the movement's heyday. OC Weekly wrote an interesting biography of her when she first came to national prominence. (Taitz lives in Southern California.)

In 2009, Taitz embraced the idea that both parents of a natural-born citizens also had to also be citizens. This isn't quite the same as having been born in the U.S., but it's pretty close. To back her position, she pointed to another blog post that suggests that the Constitution's standard necessarily follows from Vattel's "Law of Nations," a 1758 predecessor to our nation's founding document. That book says that "NATURAL-BORN CITIZENS, are those born in the country, of parents who are citizens." But "Law of Nations" is not the Constitution, and the argument for applying this standard is ... strained.

That same argument was applied by Taitz again more recently. In a long blog post, Taitz argued that Cruz isn't eligible specifically because he doesn't meet that "Law of Nations" standard. But she's given up on fighting it. "[D]ue to the courts unwillingness to adjudicate Obama’s lack of eligibility on the merits," she wrote, "now anyone can be the US President."

Good news for both Cruz and Trump.