Republican presidential front-runner Donald Trump escalated his ongoing battle with GOP rival Ted Cruz today, calling the Texas senator "unstable" and threatening him with legal action. The billionaire businessman also called on the Republican National Committee to "intervene" in the matter.

"Ted Cruz is a totally unstable individual," said Trump in a statement, according to the Hill. "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."

However, Trump was not threatening a lawsuit over Cruz's words, but rather his citizenship - a question that has routinely been raised in recent months.

"One of the ways I can fight back is to bring a lawsuit against him relative to the fact that he was born in Canada and, therefore, cannot be President," said Trump, according to Business Insider. "If he doesn't take down his false ads and retract his lies, I will do so immediately. Additionally, the RNC should intervene and if they don't they are in default of their pledge to me. "

Later, in a press conference, Trump reiterated his position that he has "never ever met a person that lies more than Ted Cruz." He added: "What Cruz says is incredible. I've never seen anything like it."

For its part, the Cruz campaign responded by saying Trump is lashing out "with name-calling and falsehoods."

"To be fair to Donald, he doesn't know what he actually believes so he can't really be expected to defend himself with reason and facts," Rick Tyler, the communications director for Cruz's campaign, told CNN. "He can't be held to account for his own words because he likely doesn't remember from one day to the next what he says about anything because he has no core set of beliefs."

Cruz also weighed in on the issue, adding Marco Rubio to the mix after the Florida senator also accused Cruz of lying about his record.

"It is not being honest or candid for either Marco Rubio or Donald Trump to pretend that their records are different than what they are and simply to yell and scream at anyone who points to the words that have come out of their own mouths," said Cruz, according to the Hill.