Donald Trump on Friday afternoon threatened to sue Sen. Ted Cruz (R-TX) over his Canadian birth unless the Texas senator doesn’t “clean up his act.”

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

Trump was quick to question Cruz’s eligibility to run for president, given that he was born in Canada to an American mother. The real estate mogul raised the question back in March 2015 when Cruz announced his presidential bid, and then brought it up again in January. Cruz has maintained that he is eligible to be president since he became a U.S. citizen at birth.

Trump’s tweet on Friday followed several others earlier in the day attacking Cruz. He bashed Cruz after the Texas senator said that Trump accepted the Supreme Court ruling legalizing same-sex marriage.

Lying Cruz put out a statement, “Trump & Rubio are w/Obama on gay marriage.” Cruz is the worst liar, crazy or very dishonest. Perhaps all 3? — Donald J. Trump (@realDonaldTrump) February 12, 2016

How can Ted Cruz be an Evangelical Christian when he lies so much and is so dishonest? — Donald J. Trump (@realDonaldTrump) February 12, 2016

Trump on Thursday accused Cruz of using “push polls” to attack him in South Carolina, but Cruz has said that his campaign is not behind the automated polls going out to voters.