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Donald Trump threatened to bring a lawsuit over Ted Cruz's Canadian birth unless the Texas senator apologizes to him for repeated "lies."

At a news conference from South Carolina, Trump called Cruz an "unstable guy," saying the senator has been repeatedly lying to voters about Trump's positions.

He said he felt better about calling Cruz a liar after Sen. Marco Rubio said the same thing.

"He'll apologize. But I don't want an apology after the election. I want the apology before. And if he doesn't, I'm gonna bring a lawsuit because in my opinion - based on what I've learned in the last two to three days from very top lawyers - he doesn't even have the right to serve as president or even run as president because he was born in Canada. So I will bring that lawsuit if he doesn't apologize," said Trump, adding that it would be filed "pretty quick."

Trump agreed that "in many respects" the campaign has now become "personal." He argued that Jeb Bush also attacks him, but doesn't come after him with lies like Cruz has been doing.

"I've never met a person who lies more than him," said Trump.

Trump said the reason that he called the press conference was to discuss his opposition to closing Guantanamo Bay. 

Earlier in the day, Cruz claimed on Fox and Friends that if Trump wins the election, he will nominate a liberal Supreme Court justice that will not protect gun rights.

Cruz responded minutes later, tweeting out a YouTube video in which he said that Trump and Rubio "scream liar" when someone points out their record.

Watch the remarks above and Cruz's response below.

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