HANAHAN, S.C. — Republican presidential candidate Donald Trump on Monday threatened to sue rival Ted Cruz, challenging his eligibility to serve in the White House unless the Texas senator stops airing what Trump calls “false ads” and retracts what the billionaire real estate mogul called a series of lies.

With less than a week to go before South Carolina’s primary, the Republican front-runner also reiterated that the 9/11 attacks happened during President George W. Bush’s time in office – an apparent attempt to overshadow the former president’s Monday campaign appearances on behalf of his brother, Jeb Bush.

Trump focused much of his criticism at Cruz, however.

“If he doesn’t take down his false ads and retract his lies,” Trump said in a statement he will immediately file a lawsuit challenging Cruz’s eligibility to serve as president.

Trump has previously said a federal court should decide whether Cruz meets the constitutional requirement of being a “natural-born citizen” to serve as president. Cruz was born in Canada to an American mother, and legal experts have said he meets that test.

The statement took issue with Cruz’s recent suggestions that Trump would appoint liberal judges to serve on the Supreme Court, is opposed to abortion rights and would undermine the Second Amendment.

“Ted Cruz is a totally unstable individual. He is the single biggest liar I’ve ever come across, in politics or otherwise, and I have seen some of the best of them. His statements are totally untrue and completely outrageous,” Trump said in the statement.

Cruz responded to Trump during a question-and-answer session after an appearance at a gallery in Camden, characterizing Trump as nervous about his standing in the state.

“Today Donald Trump held a press conference. He apparently lost it. I mean, he was just going on and on about how I was the most horrible person in the world because I keep repeating the things he said,” Cruz said to laughs. “And it’s an amazing thing. Have you noticed how rattled Donald gets when his numbers start going down? He gets very, very upset.”

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