Donald Trump has been suggestion for months that Democrats might sue Ted Cruz over his citizenship eligibility to be president, but now it's the Republican frontrunner who is talking about legal action against his closest rival.

Trump tweeted the threat of a lawsuit Friday after Cruz released an attack ad against Trump, suggestion the billionaire "had no heart" while building his real estate empire.

Also see: Ted Cruz Ad: Donald Trump 'Doesn't Have No Heart'

"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.

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.

-- Donald J. Trump (@realDonaldTrump) February 12, 2016

Cruz was born in Canada and his parents -- including his American-born mother -- moved to Texas when he was four. Cruz claims that makes him a "natural born U.S. citizen," as required for a president under the U.S. Constitution. Trump and a number of noted constitution scholars have questioned that conclusion.