Donald Trump. REUTERS/Jonathan Bachman Republican presidential front-runner Donald Trump just threatened to sue US Sen. Ted Cruz (R-Texas) over his eligibility to be president.

"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 warned Friday on Twitter.

Cruz was born in Canada to an American mother, and most legal experts believe that he meets the constitutional requirement that presidents are "natural-born" citizens.

But the courts have not formally ruled on the issue, and Trump has repeatedly raised doubts about his opponent's eligibility for the Oval Office. Trump frequently warns that Democrats would sue the senator if he got the nomination. In January, he jokingly floated the idea of suing Cruz himself.

The senator — who has repeatedly mocked and dismissed Trump's concern over his birthplace — launched an attack ad on Thursday that accused Trump of having "a pattern of sleaze stretching back decades." The commercial focused on Trump's support for eminent domain, the government's power to seize private property.

Among other things, Trump accused Cruz of lying when the senator denied being behind "push polls" apparently conducted on his behalf. According to The Washington Post, a robocall in South Carolina criticized the non-Cruz candidates in the race.

"Cruz caught cold in lie after denial of push polls ... How can he preach Christian values?" Trump tweeted on Thursday.

A Cruz campaign official responded to Trump's legal threat on Friday by suggesting that the mogul didn't understand the law: