| Jocelyn Augustino, American Media Institute

Trump attacks Cruz 'lies,' threatens legal action

The feud between billionaire real estate developer Donald Trump and Texas Sen. Ted Cruz continues to escalate, with Trump promising that "If Ted is going to continue to lie with such desperation, I have no choice but to fight back."

The bad blood between the two grew worse as the Cruz campaign began airing a new television ad in the state targeting Trump for his past statements in support of abortion rights.

Focusing on a "Meet the Press" interview between Trump and the late Tim Russert, the ad quotes Trump as saying he is "very pro-choice." When Russert presses Trump on whether he would support ending partial-birth abortions, Trump says "no," and states he is "pro-choice in every respect."

It's not a new line of attack for Cruz.

At Saturday's Republican debate in Greenville, South Carolina, Cruz and Trump tangled over federal funding for Planned Parenthood. Cruz said Trump "supports taxpayer funding for Planned Parenthood," and that Trump "supported it when [Congress was] battling over defunding Planned Parenthood."

Trump retorted that Cruz's assertions were "a lot of lies," and that Planned Parenthood "does do wonderful things, but not as it relates to abortion."

On the trail Monday, Trump broadened his response to Cruz, calling the Texas senator "the single-biggest liar I’ve ever come across, in politics or otherwise."

In his press release, Trump says that, contrary to Cruz's assertions, he would nominate "a great conservative" to fill the Supreme Court vacancy left by the death of Associate Justice Antonin Scalia.

On abortion, Trump said, "Cruz says I am pro-choice, when in fact I am staunchly pro-life and have been for a long time. 

"Like Ronald Reagan, on many issues I have evolved."

Trump threatened to sue Cruz over the ad, stating he would "bring a lawsuit against him relative to the fact that he was born in Canada and therefore cannot be president."

"If he doesn’t take down his false ads and retract his lies," Trump said, "I will do so immediately."

Trump also took aim at the Republican National Committee.

"Additionally," he said, "the RNC should intervene, and if they don’t they are in default of their pledge to me."

Republican Party leaders made an agreement with Trump last year to see that Trump was treated fairly in the nominating process in return for his promise not to mount a third-party bid for the White House.