In separate interviews with CNN, Trump and Cruz squared off over the businessman's comments -- reported Tuesday in The Washington Post -- that the senator's birth in Canada could pose a "big problem." Trump told CNN's Wolf Blitzer that Cruz, whose mother was a U.S. citizen, should go to court and ask a judge to rule that he's eligible to run for president.
"How do you run against the Democrat, whoever it may be, and you have this hanging over your head if they bring a lawsuit?" Trump said in an interview that aired on "The Situation Room."
Trump said Cruz should to ask a judge for a "declaratory judgment" that Trump said would protect Cruz against any future questions about his eligibility that could come in a general election.
He also claimed that it was The Washington Post -- not him -- that raised the issue.
"This was not my suggestion. I didn't bring this up. A reporter asked me this question," Trump said. He also repeated his assertion that Cruz has a Canadian passport, which Cruz denied during a separate interview Wednesday with CNN's Dana Bash.
Trump added: "I'm doing this for the good of Ted ... I like him. He likes me."
On Thursday, Trump kept up the drumbeat via Twitter, referencing the fact that Arizona Sen. John McCain -- who once called Cruz a "wacko bird" -- also said Cruz's eligibility was a "legitimate question."
"It was a very wise move that Ted Cruz renounced his Canadian citizenship 18 months ago. Senator John McCain is certainly no friend of Ted!," Trump tweeted.
He also offered Cruz his "free legal advice."
".@SenTedCruz Ted--free legal advice on how to pre-empt the Dems on citizen issue. Go to court now & seek Declaratory Judgment--you will win!" he tweeted.
For his part, Cruz said he is certain that he never owned a Canadian passport, pushing back sharply on the idea that there is any legal controversy whatsoever.
"Of course not," Cruz told Bash, as his campaign bus traveled from Sibley to Spirit Lake close to the Iowa-Minnesota border. "Yes, I'm sure. The media, with all due respect, love to engage in silly sideshows. We need to focus on what matters."
He went on: "The legal issue is straightforward," he said, calling it a "non-issue." "Listen, the Constitution and the laws of the United States are straightforward. The very first Congress defined the child of a U.S. citizen born abroad as a natural-born citizen."
Experts largely agree that Cruz
, who was born an American citizen thanks to his mother's nationality, is eligible to serve as president. The U.S. Constitution requires that presidents be "natural born citizens," which Cruz is alleged to not be, since he was born in Canada. Cruz has the backing of most legal experts
and U.S. case law, according to a 2011 Congressional Research Service study.
Cruz's unequivocal answer comes as he is neck-and-neck with Trump in Iowa just weeks before that state's first-in-the-nation caucuses.
Cruz is in the middle of a six-day, 28-event bus tour, reveling in criticism from most of his fellow GOP rivals at nearly every stop.
Cruz has led Trump in the most recent batch of polling in Iowa. Trump, who at recent events has pondered aloud about the fact that he could lose Iowa to Cruz, has nevertheless recently begun intentionally poking at his faith, nationality, and as of Tuesday, his citizenship.
"I'd hate to see something like that get in his way," Trump told The Washington Post, saying that Cruz could get tied up in a lawsuit if elected. "But a lot of people are talking about it and I know that even some states are looking at it very strongly, the fact that he was born in Canada and he has had a double passport."
Cruz has defended himself by pointing to Arizona Sen. John McCain, Mitt Romney's father George Romney and former Arizona Sen. Barry Goldwater as examples of Republican presidential candidates who ran for office even though they were born outside the U.S.
Perhaps to allow the controversy to continue, House Minority Leader Nancy Pelosi wouldn't definitively say whether she thought Cruz was "naturally born" citizen when asked about it Thursday at her weekly press conference.
She waved off weighing in saying it was "inside baseball" and a constitutional issue. But she also made a point of pointing out that McCain was born in U.S. territory versus Cruz, who was born in Canada, saying "perhaps there is a distinction."