Ted Cruz. REUTERS/Mark Kauzlarich Donald Trump hasn't always been on the same page as constitutional lawyers. But at least a few are starting to provide some validation to Trump's continued questions about US Sen. Ted Cruz's (R-Texas) eligibility to serve as US president.

In the last week, the Republican presidential front-runner has said repeatedly that Cruz should seek a legal ruling over whether Cruz — who was born in Canada to an American mother and a Cuban father — is, in fact, a "natural-born citizen."

"He was born in Canada. If you know and when we all studied our history lessons, you are supposed to be born in this country, so I just don't know how the courts would rule on it. It's an additional hurdle that he has," Trump said last week.

On Wednesday, Trump claimed that "sadly," there's "no way that Ted Cruz can continue running" for president unless he "can erase doubt on eligibility."

A handful of prominent legal scholars have come to agree with the premise of Trump's question.

Although there seems to be no question that Cruz's mother's status as an American citizen ensured that he was naturalized at birth, Article II of the US Constitution requires that a president be a "natural-born citizen." Some constitutional-law experts have argued that the framers likely included this language to refer to only those born within US borders.

Writing in The Boston Globe, Laurence Tribe — who taught US President Barack Obama and Cruz at Harvard Law School — said that it's unclear whether a more modern interpretation of the Constitution allows a person of Cruz's status to run.

"The constitutional definition of a 'natural-born citizen' is completely unsettled, as the most careful scholarship on the question has concluded," Tribe wrote.

He claimed that Cruz's own rigid interpretation of the Constitution on many issues would likely disqualify the senator.

"Needless to say, Cruz would never take Donald Trump's advice to ask a court whether the Cruz definition is correct, because that would in effect confess doubt where Cruz claims there is certainty," he wrote.

Cass Sunstein, a professor at Harvard Law School and a former Obama administration official, largely echoed those sentiments.

He wrote in Bloomberg View that although a 1790 law suggested that Cruz should be eligible, significant legal ambiguity exists within the law itself and the Constitution.

"On the merits, I agree with Cruz: The Naturalization Act of 1790 counts in his favor, and because he was a US citizen at birth under US law, the better view is that he is natural born," Sunstein wrote. "But University of San Diego constitutional specialist Michael Ramsey, a former law clerk to Justice Antonin Scalia, put it well: 'It's a mystery to me why anyone thinks it's an easy question.'"

And Mary Brigid McManamon, a constitutional-law professor at Widener University's Delaware Law School, argued in The Washington Post on Tuesday that Trump was on to something because of a key distinction.

"Cruz is, of course, a US citizen. As he was born in Canada, he is not natural-born," she argued.

Donald Trump and Ted Cruz. AP

Cruz on Tuesday provided his hardest-hitting defense of Trump's questions about his eligibility. He said that Tribe is a left-leaning legal scholar, and claimed he is a supporter of former US Secretary of State Hillary Clinton. Tribe himself has not appeared to officially offer Clinton his support.

"I do think it's interesting that Hillary's key supporters are doing everything they can to echo Donald's attacks on me," Cruz said in an interview on Wednesday.

He continued: "It may be driven by the fact that the polling shows that Donald loses to Hillary, and loses by a pretty big margin. But I beat Hillary, and that might have the Hillary folks a little concerned."

Cruz also said that Trump "embodies New York values" in a radio-show interview.

Some conservatives have agreed with Cruz, suggesting that it's not a coincidence that Tribe and Sunstein, both considered left-leaning legal experts, would raise questions about Cruz's eligibility.

"I think it's the worst kind of self-serving crap that really turns people off of politics in this country," veteran GOP strategist Liz Mair said of the questions about Cruz's eligibility. "Some people will say or do literally anything for their preferred guy to win, or for an opponent they dislike to lose, and the fact is, Cruz has a lot of haters and detractors on both sides of the aisle."

Ted Cruz. AP

To that end, Cruz hasn't received backing from much of the same Republican establishment that provided former Republican nominee and Sen. John McCain (R-Arizona) with support when questions surrounded his 2008 bid. McCain, in fact, suggested that the issue was "worth looking into."

And for their part, Democrats have seemed to enjoy the squabble over Cruz's eligibility.

White House press secretary Josh Earnest said last week that he found it "ironic" that Republicans would consider nominating Cruz considering the so-called birther attacks, fueled by Trump, that prompted Obama to release his long-form birth certificate in 2011.

"It would be quite ironic if after seven or eight years of drama around the president's birth certificate, if Republican primary voters were to choose Sen. Cruz as their nominee, somebody who actually wasn't born in the United Sates and who only eight months ago renounced his Canadian citizenship," Earnest said last week during his daily press briefing.

Earnest smiled when asked if the president was enjoying watching Trump's questions about Cruz's eligibility following the president's own long battle with Trump over his place of birth.

"I don't know if he does, but I sure do," he said.

And polling has shown that the tactic could pay off for Trump in Iowa, the key first nominating state, where Trump and Cruz are currently running neck-and-neck.

According to a Public Policy Polling survey released on Tuesday, only 32% of likely Republican caucus-goers in Iowa said that someone born outside the US should be allowed to serve as president. The same poll found that 47% of respondents think that being born outside the US should make someone ineligible.