Donald Trump has long questioned whether Sen. Ted Cruz, who was born in Canada, is eligible to be president. Now, Trump is threatening to sue Cruz over it.
Trump maintains Cruz may not be a natural born citizen, one of a few qualifications to be president listed in the Constitution. Trump tweeted Friday he has standing to sue Cruz over the issue.
Cruz has defended himself from the "birther" claim that he's disqualified from the office, including in a presidential debate in January.
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"If @TedCruz doesn’t clean up his act, stop cheating, & doing negative ads, I have standing to sue him for not being a natural born citizen," Trump tweeted Friday afternoon.
There was no immediate response on Ted Cruz's Twitter feed, and his campaign did not reply to a message requesting comment.
The president must be a "natural born citizen," 35 years old and a 14-year resident of the nation, according to Article II of the Constitution. The 25th Amendment establishes the two-term limit.
Cruz was born a U.S. citizen because his mother was an American living in Canada. Some constitutional scholars – and Trump – have questioned whether the circumstances of Cruz's birth meet the "natural born citizen" requirement.
A veteran attorney in Houston, Cruz's hometown, has already challenged the senator's eligibility in a federal court, one of several suits brought against Cruz.
At the Jan. 17 debate, Trump suggested Cruz voluntarily submit the question to the court system, saying "there's a big question mark over your head." Trump said he wouldn't sue, but Democrats surely would if Cruz became the Republican nominee for president.
But Cruz countered that he was born a citizen, and that two Republicans born outside of the country had already run for president, including John McCain.
"I'm happy to consider naming you as vice president," Cruz quipped at the time. "And if you happen to be right you can get the top job at the end of the day."
Harvard Law professor Einer R. Elhauge wrote in a January op-ed that Cruz is ineligible to run, based on a straight reading of the Constitution, but said a fixed rule should be implemented to clarify.
In an interview Friday, Elhauge said it's unlikely Trump would have standing to sue the Cruz campaign in federal court, because "federal standards are pretty narrow."
"A candidate would have to have a unique injury to them in order to sue," he said.
But the rules for standing in state courts are broader, he said, so "any candidate or voter can challenge" Cruz.
Now that the field of Republican contenders has narrowed to six, each candidate may constitute a threat to the others' vote totals, and therefore a "unique injury," according to Elhauge. Trump would probably need a state election official to speak up for his claim.
Professor Rick Hasen of the University of California at Irvine told NBC News that it would be unlikely that Trump would have standing in a direct case against Cruz, but that he could sue election officials for allowing Cruz on the ballot.
"And certainly if Cruz were excluded from the ballot by a registrar who said Cruz is ineligible, Cruz would have standing to sue over that," Cruz said.
Patrick Smith contributed to this report.