GOP presidential front-runner Donald Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE on Monday threatened rival Ted Cruz Rafael (Ted) Edward CruzLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list 'Parks and Rec' cast members hosting special reunion to raise money for Wisconsin Democrats MORE with legal action if the Texas senator does not “take down his false ads and retract his lies.”

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“Ted Cruz is a totally unstable individual,” Trump said in a statement. “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.”

"Cruz has become unhinged and is lying with the hopes that his statements will go unchecked until after the election and he will save his failing campaign," Trump added.

Trump said one way he can fight back is to bring a lawsuit against Cruz “relative to the fact that he was born in Canada and therefore cannot be President.”

Trump also called on the Republican National Committee to intervene, saying the RNC would be “in default of their pledge to me” if it doesn’t act.

Cruz fired back on Twitter, saying he is pointing out Trump's "actual positions."

You cannot simply scream “liar” when someone points out your actual positions: https://t.co/DI5HnSln62 — Ted Cruz (@tedcruz) February 15, 2016

At a press conference on Monday, Trump reiterated his threat, again calling Cruz "unstable" and saying he will bring a lawsuit if Cruz doesn't "straighten his act out."

"What Cruz says is incredible. I've never seen anything like it," he told reporters.

Trump said he's been accused by Cruz of being against the Second Amendment, of being in favor of Obamacare and of being pro-abortion — all of which Trump said isn't true.

Trump repeatedly called Cruz a liar and vowed to not let the senator get away with his statements.

"I have never ever met a person that lies more than Ted Cruz," he said.

"I have a very good lawyer, already hired," Trump added.

This report was updated at 3:02 p.m.