Donald Trump, left, watches as Ted Cruz speaks during the Dec. 15 CNN Republican presidential debate at the Venetian Hotel & Casino in Las Vegas. (John Locher/AP)

Donald Trump said in an interview that rival Ted Cruz’s Canadian birthplace was a “very precarious” issue that could make the senator from Texas vulnerable if he became the Republican presidential nominee.

“Republicans are going to have to ask themselves the question: ‘Do we want a candidate who could be tied up in court for two years?’ That’d be a big problem,” Trump said when asked about the topic. “It’d be a very precarious one for Republicans because he’d be running and the courts may take a long time to make a decision. You don’t want to be running and have that kind of thing over your head.”

Trump added: “I’d hate to see something like that get in his way. 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 responded to Trump’s comments on Twitter later Tuesday evening by referring to an iconic episode of the sitcom “Happy Days,” in which the character Fonzie jumps over a shark on water skis. The image has become a symbol of something shopworn and overdone.

Trump’s remarks — part of a backstage interview before a rally here Monday night — come as Cruz is rising as a serious threat in the presidential campaign, especially in Iowa, where some polls have shown Cruz eclipsing the billionaire mogul. The two have had a cordial and at times even friendly relationship over the past year, but they are competing intensely for the support of conservatives as the Feb. 1 Iowa caucuses draw near.

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There have been recent signs of tension. At a rally last month in Iowa, Trump told voters of Cruz: “Just remember this — you’ve got to remember, in all fairness, to the best of my knowledge, not too many evangelicals come out of Cuba, okay? Just remember that . . . just remember.”

In the interview with The Washington Post, Trump said he was providing a candid assessment of his leading opponent rather than initiating a personal attack and reviving the “birther” debate that he once led against President Obama. He repeatedly said he is hearing chatter on the topic among voices on the right. “People are bringing it up,” he said.

Trump has veered from shrugging off the issue to raising more questions himself. In an interview with ABC News in September, Trump said he did not think Cruz’s birthplace was an issue. “I hear it was checked out by every attorney and every which way and I understand Ted is in fine shape,” he said.

But months earlier in Iowa, Trump told reporters that it could be a “difficult problem.”

“He’s a friend of mine. I have great respect for him. . . . certainly it’s a stumbling block, and he’s going to have to have it solved before he goes too far,” Trump said, according to the Dallas Morning News.

Speaking late Tuesday in Sioux Center, Iowa, Cruz laughed off questions about Trump’s comment, saying he would let his campaign’s “Happy Days” tweet speak for itself.

When pressed, Cruz turned it back to the media, saying the focus should be on substantive issues.

Republican presidential candidate Ted Cruz has been complimenting one of his 2016 rivals, businessman Donald Trump, for months. Now, Trump is warming up to Cruz, too. Here's what the two men have had to say about one another on the political trail. (The Washington Post)

“And one of the things that the media loves to do is gaze at their navels for hours on end by a tweet from Donald Trump or from me or from anybody else. Who cares?” he said. When asked why he would tweet a video clip, he said: “Why do it? Because the best way to respond to this kind of attack is to laugh it off and move on to the issues that matter.”

Despite this, Cruz maintains he still likes Trump and doesn’t intend to throw insults.

The Constitution requires a president to be a “natural-born citizen.” Anyone born to a U.S. citizen is granted citizenship under U.S. law, regardless of where the birth takes place, as long as the citizen parent has resided in the United States or its territories for a certain period of time. At the time of Cruz’s birth, the required period was at least 10 years, including five years after the age of 14.

Cruz’s mother was a U.S. citizen when he was born in Calgary in 1970; his father was born in Cuba. Cruz has long said that because his mother is a citizen by birth, he is one as well and fits under the definition of a natural-born citizen. Since his election to the Senate, Cruz has released his birth certificate and renounced his Canadian citizenship.

Legal scholars agree that Cruz meets the Constitution’s natural-born citizenship requirement, though it is untested in the courts.

Several previous presidential candidates have run for office with similar backgrounds, such as Sen. John McCain (Ariz.), the 2008 Republican nominee, who was born in the Panama Canal Zone to U.S. citizens.

In the interview, Trump alluded to an ongoing lawsuit in Vermont, where a man is trying to keep three Republican presidential candidates, including Cruz, off the ballot. According to the Rutland Herald, the lawsuit names state officials as defendants.

Trump has long flirted with “birtherism,” questioning Obama’s love of country and legal claim to the presidency. He supported efforts to investigate Obama’s birth in Hawaii and often suggested that the president was born outside the country.

Trump’s crusade reached its zenith in 2011, when Obama felt obliged to publicly release his long-form birth certificate. The president then mocked Trump over the issue at the White House Correspondents’ Association dinner that year. Since then, Trump has quieted his speculation about Obama’s birth, while still declining to accept Obama’s legitimacy as president.

Katie Zezima in Sioux Center, Iowa, contributed to this report.