Sen. Ted Cruz (R-TX) and businessman Donald Trump went toe-to-toe in a thrilling exchange at the GOP Debate in Charleston, South Carolina on Thursday evening as they debated Cruz’s constitutional eligibility for the presidency.

The fight, which opened the second round of questions, was triggered by moderator Neil Cavuto, who asked Cruz directly whether the fact that he had been born in Canada meant he was not a “natural born citizen.”

Then, this:

CAVUTO: Senator Cruz. Now you are, of course, a strict constitutionalist. No one would doubt that. And, as you know, the U.A. Constitution says only natural born citizens are elgible for the office of President of the United States. Stop me if you heard this before. Now you were born, you were born in Canada to an American mother. so you were and are considered an American citizen. But that fellow next to you, Donald Trump–and others–have said being born in Canada means you are not natural born and that has raised questions about your eligibility. Do you want to try to close this topic once and for all tonight?

CRUZ: (Laughs) Well, Neil, i’m glad we’re focusing on the important topics of the evening.

(Laughter, Applause)

Back in September, my friend Donald said that he’d had his lawyers look at this from every which way, and there was no issue there. There was nothing to this “Birther” issue.

(Laughter)

Now since September, the Constitution hasn’t changed.

(Laughter)

But the poll numbers have.

(Applause)

And I recognize, I recognize that Donald is dismayed that his poll numbers are falling in Iowa. But the facts and the law here are really quite clear. Under longstanding U.S. law, the child of a U.S. citizen born abroad is a natural born citizen. If a soldier has a child abroad, that child is a natural born citizen. That is why John McCain, even though he was born in Panama, was eligible to run for president. If an American missionary has a child abroad, that child is a natural born citizen. That’s why George Romney, Mitt’s dad, was eligible to run for president, even though he was born in Mexico. At end of the day the legal issue is quite straightforward but I would note that the birther theories that Donald has 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.

(Applause)

Because, because, Donald’s mother was born in Scotland. She was naturalized. Now Donald– —

TRUMP: But I was born here.

CRUZ: On the issue of citizenship, Donald–

TRUMP: Big difference.

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

(Laughter)

TRUMP: Because it wouldn’t work.

CRUZ: You’re an American. As is everybody else on this stage. And I would suggest we focus on who’s best prepared to be commander-in-chief, because that’s the most important question facing the country.

(Applause)

CAVUTO: Mr. Trump…you raised it, because of his rising poll numbers.

TRUMP: Let me tell you something, and you know, because you saw the numbers yourself, NBC, Wall Street Journal came out with a poll headlined, “Trump Way up, Cruz Going Down.” I mean, so don’t (Boos)–see, you can’t, you can’t, they don’t like the Wall Street Journal, they don’t like NBC, but I like the poll. And frankly, it just came out–and in Iowa now, as you know, Ted, in the last three polls I’m beating you. So you shouldn’t misrepresent how well you’re doing with the polls.

(Applause)

You don’t have to say that. In fact I was all for you until you started doing that. Because that’s a misrepresentation. Number one. Number two, this isn’t me saying it. I don’t care. I think I will win fair and square. I don’t have to win this way. Thank you. Laurence Tribe and numerous–from Harvard, of Harvard–said that there is a serious question as to whether or not Ted can do this, okay? There are other attorneys that feel–and very, very fine constitutional attorneys–that feel that because he was not born on the land, he cannot run for office. Here’s the problem. We’re running, we’re running. He does great. I win. I choose him as my vice-presidential candidate and the Democrats sue, because we can’t take him along for the ride. I don’t like that. Okay?

(Laughter)

The fact is, and if, for some reason he beats the rest of the field, he beats the rest of the field who is–

(Boos)

See, they don’t like that. They don’t like that. (Boos) They don’t like that “he beats rest of the field,” because they want me. If for some reason, Neil, he beats rest of the field, I already know the Democrats are going to be bringing a suit. You have a big lawsuit over your head while you’re running and if you become the nominee, who the hell knows if you can even serve in office? So you should go out, get a declaratory judgment. Let the courts decide and —

CAVUOT: Why are you saying this now? Why are you raising this issue now?

TRUMP: Because now he is doing a little bit better.

(Laugher, Boos)

No, I didn’t care if he, no, it is true. Ne never had a chance. Now he is doing better. He’s got probably a 4 or 5% chance.

(Boos, Applause)

CAVUTO: Thank you, Mr. Trump.

CRUZ: Neil–

TRUMP: The fact his, there is a big overhang. There is a big question mark on your head. And you can’t do that to the party. You really can’t. You can’t do that to the party. You have to have certainty. Even if it was a 1% chance, and it’s far greater than 1%, because he wasn’t born–you have great constitutional lawyers say you can’t run. And you know, I’m not bringing a suit, I promise, but the Democrats are going to bring a lawsuit. And you can’t–and you have to have certainty. You can’t have a question. I can agree with you or not. But you can’t have a question over your head.

CAVUTO: Senator, want to respond?

CRUZ: Listen, I spent my entire life defending the Constitution before the U.S. Supreme Court, and I’ll tell you I’m not going to be taking legal advice from Donald Trump.

(Applause)

TRUMP: You don’t have to. take it from Laurence Tribe. Take it from your professor. take it from your own professor.

CRUZ: But I will tell you, the chances of any litigation proceeding and succeeding on this, are zero. Now Mr. Trump–

TRUMP: That’s wrong. It’s wrong.

CRUZ: But Mr. Trump is very focused on Larry Tribe. Let me tell you who Larry Tribe is. Left-wing judicial activist Harvard Law professor, Al Gore’s lawyer in Bush v. Gore. He is a major Hillary Clinton supporter. There is a reason why Hillary supporters are echoing Donald’s attacks on me. Because Hillary wants to face Donald Trump in the general election.

TRUMP: He is not the only one. Not only one. There are many lawyers.

CRUZ: And I’ll tell you what, Donald, you very kindly just a moment ago offered me the VP slot. I’ll tell you what. If this all works out I’m happy to consider naming you as VP, and so if you happen to be right, you could get the top job at end of the day.

(Applause)

TRUMP: No. I think–I think if it doesn’t–I like thatt, I would consider it, but I think I will go back to building buildings if it doesn’t work out.

CRUZ: Actually, I’d love you to build a wall.

TRUMP: I have a feeling it’s going to work out, actually.

(Rush transcript)