Sen. Ted Cruz, R-Texas, campaigns at Penny's Diner in Missouri Valley, Iowa. (AP Photo/Nati Harnik, File)

SPIRIT LAKE, Iowa -- Sen. Ted Cruz (R-Tex.) attempted to drive a stake through a persistent -- and legally dubious -- rumor recently stoked by Donald Trump. Speaking to reporters before an event in the city of Rock Rapids, Cruz said that his birth in Canada to a Cuban father and American mother did not change the fact that he was a natural born citizen.

"It's settled law," Cruz said, to a question from Todd Gillman of the Dallas Morning News. "The child of a U.S. citizen, born abroad, is a U.S. citizen. People will continue to make hay of it as a political matter, but as a legal matter it's quite straightforward. It's occurred many times in history. John McCain was born in Panama. He was a natural born citizen because his parents were citizens. George Romney: He was born in Mexico, and his parents were U.S. citizens. Actually, Barry Goldwater was born in Arizona before Arizona was a state, and yet he was a natural born citizen."

That was the most Cruz had said about the topic, a surprising echo of a conspiracy that dogged President Obama through two elections and a series of frivolous lawsuits. Cruz, unlike the president, was born outside the country. That put a lifelong constitutional scholar, someone who earned money as a teenager giving speeches about the document's greatness, arguing what the founders meant when they wrote that "no person except a natural born citizen" could walk into the White House.

In 2013, after the Dallas Morning News reported that Cruz had -- but was not making use of -- dual Canadian citizenship, the new senator told CNN that there were no serious questions about his citizenship. "I was born in 1970 in Calgary, Canada; my parents were working there in the oil and gas business," he said. "My mother was a U.S. citizen by birth born in Wilmington, Delaware, so under U.S. law, I’m an American citizen by birth."

But the theory that somehow, Cruz could be knocked out of a presidential race on a technicality, was too delicious for some people to resist. A few Democrats, like Rep. Alan Grayson (D-Fla.), suggested a turnabout-is-fair-play lawsuit might be in order, just to irritate Cruz. On the right -- i.e. among voters more inclined to like Cruz anyway -- no real movement to question his citizenship sprang up, and in at least one poll a plurality of Republicans mistakenly assumed he was born in the United States.

Trump's Tuesday and Wednesday comments about the "issue" to the Washington Post and Fox News, found no real takers on the Cruz campaign trail. The candidate never goes without a mention of his father's story -- an immigrant from Cuba, finding freedom in the United States.

"I've heard that nonsense from the news, from Trump," said Jean Greely, 67, at a Cruz town hall in Spirit Lake. "I don't believe it."

"I read about that once," said Jim Wells, a 73-year-old accountant in Rock Rapids, "but obviously he's qualified to run."

Over three busy stops with more than a dozen total questions, Cruz did not get a single audience inquiry about his citizenship. In an interview with ABC News between stops, he batted away a question about Trump's record by returning to his usual answer on any of the mogul-candidate's insults.

"It's not surprising that a lot of the folks in the media would love to see Donald and I get in a big food fight," said Trump, "and the person who would benefit the most by that is Hillary Clinton. And I'm certainly not going to engage in that, and I hope Donald won't either."

Donald Trump and Ted Cruz avoided going after each other on the debate stage or the campaign trail – until this week. The Fix's Chris Cillizza explains why the Trump-Cruz dynamic isn't going away any time soon. (Peter Stevenson/The Washington Post)