First Donald Trump took aim at rival Ted Cruz's evangelical credentials. Now he's questioning whether the Canadian-born Texas senator is even eligible for the White House.

"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 of Cruz's birthplace and citizenship in an interview with the Washington Post. "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."

The return to so-called "birtherism" is nothing new for the real estate mogul, who's currently atop the polls for the Republican nomination. Back in 2011, when Trump first floated a GOP presidential run, he famously questioned whether President Obama was actually born in Hawaii. Once the president did release the long-form version of his birth certificate, Trump boasted that he was "proud of myself because I've accomplished something nobody has been able to accomplish."

After the Trump-fueled controversy over Obama's birthplace, the question over Cruz's was a natural one that's already come up. Cruz was born in Calgary, Canada, in 1970 while his parents were working in the oil industry. Though his dad is from Cuba, his mother was a U.S. citizen, having been born in Delaware. Cruz held dual U.S. and Canadian citizenship, though he renounced the latter in 2014.

Here's how NPR's Domenico Montanaro explained the historical precedent that should give Cruz a pass back in March 2015:

"The U.S. Constitution says presidential candidates have to be 'natural-born citizens.' But the Supreme Court has never weighed in with a definition, leaving it open to interpretation."It's a question that has come up before. In 2008, senators passed a resolution, making it clear, for example, that John McCain was allowed to run given that he was born on a U.S. military base in the Panama Canal Zone. Barack Obama and Hillary Clinton, both senators then, voted for it."Barry Goldwater, the 1964 GOP nominee, was born in Arizona when it was a territory — not a state. And some questioned George Romney's eligibility to run in 1968, because he was born in Mexico. Romney's parents were U.S. residents."

Legal scholars have agreed that Cruz and the other candidates before him would indeed be eligible for the White House. Neal Katyal, who was acting solicitor general in the Obama administration, and Paul Clement, who was the solicitor general under George W. Bush, wrote in the Harvard Law Review that "there is no question" Cruz is eligible and that ""Cruz has been a citizen from birth and is thus a 'natural born Citizen' within the meaning of the Constitution" and the "Naturalization Act of 1790."

The possibility that Cruz may not be eligible for the White House is something that Trump himself even dismissed last fall.

"I hear it was checked out by every attorney and every which way, and I understand Ted is in fine shape," Trump told ABC News last September of his rival's constitutional eligibility because of his birthplace.

Trump had questioned Cruz's legal qualifications before, though. In December 2014, he also told ABC, "I think he's a really nice guy, I've gotten to know him a little bit, but I think if he's born in Canada it's a problem, no question about it."

But the latest reversal comes as Cruz is seriously threatening Trump's lead in Iowa and elsewhere — especially with evangelical voters critical to winning the Hawkeye State's caucuses on Feb. 1. And it was just the latest barb he's leveled against the Texas senator, telling a crowd last week that, "In all fairness, to the best of my knowledge, not too many evangelicals come out of Cuba, OK?"

While Trump floated the birther question with both the Post and NH1 television ahead of his rally Tuesday night in Claremont, N.H., he didn't raise his doubts with the crowd. In his typical stemwinder style, Trump boasted of his advantage in self-selected polls, touted his intelligence and Ivy League education, bashed the press and took aim mostly at Democrat Hillary Clinton.

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