MASON CITY, IOWA -- Thanks to Donald Trump, the Ted Cruz birther issue emerged this week on the edges of the presidential campaign. Cruz, born in Canada to an American citizen mother and a Cuban father, is an American citizen by birth, according to U.S. law. But Trump suggested there might be a question of whether Cruz meets the constitutional requirement that the president be a "natural born citizen," and that the issue could be tied up in litigation, thus creating a problem for Republicans if Cruz were to be the party's nominee.

At a town hall in Mason City Friday morning, an audience member asked Cruz about the issue. The woman, Shawna Crabb of Ventura, Iowa, said she believes Cruz is eligible but that she just wanted to hear Cruz, a lawyer who has argued before the Supreme Court, explain it himself. Here is what Cruz said:

Sure, I'm happy to. The law is straightforward on this. Under the Constitution, in order to be president, you must be a natural born citizen. Now, U.S. law has been clear from the very first days of this country that the child of a U.S. citizen born abroad is a natural born citizen. Indeed, the very first Congress, when it passed the first laws on citizenship, defined the child of a U.S. citizen born abroad as a natural born citizen. They used exactly those words. Now mind you, many members of the first Congress were themselves Framers who had been at the constitutional convention who had used the exact same words in the Constitution.

This is an issue that has come up many times in American history. For example, many of us know John McCain was born in Panama. But he was a U.S. citizen because both of his parents were U.S. citizens. He was a citizen by birth. Likewise, George Romney, Mitt's dad, who ran for president in '68, was born in Mexico. His parents were Mormon missionaries, and yet he was a U.S. citizen by birth because his parents were citizens. And the third example, interestingly enough, is Barry Goldwater. Goldwater was born in Arizona before Arizona was a state, it was just a territory. And so he was a U.S. citizen by birth by virtue of the fact that his parents were citizens.

And so as a legal matter, the question is quite straightforward. My mother was born in Wilmington, Delaware. She is a citizen by birth, so I became an American citizen by virtue of being born to her. I have never breathed a breath of air on this planet when I was not a U.S. citizen. I've never been naturalized. It was the process of being born that made me a U.S. citizen.

Cruz went on to attribute the appearance of the question to his political opponents. But on the core issue, Cruz's fellow legal experts agree with him that being a citizen by birth is the equivalent of the Constitution's "natural born citizen." It's not clear whether his explanation will stop the questions, but Cruz has now said pretty much everything that can be said on the matter.