Donald Trump to Ted Cruz: I'll Sue You

Donald Trump on Monday offered presidential rival Ted Cruz an ultimatum, threatening to sue Cruz over his eligibility to serve in the White House unless the Texas senator stops airing what Trump calls "false ads" and apologizes for what the billionaire real estate mogul called a series of lies about his positions. With less than a week to go before South Carolina's Republican primary, the GOP 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. The new attacks came as the race entered an increasingly nasty phase, with numerous negative ads airing on local television following an unusually caustic debate this past weekend.

Some of the harshest ads have been aimed at Trump, often using the political newcomer's past words to illustrate his evolving position on issues including abortion and gun rights. Trump also took aim Monday at the Republican establishment, accusing the Republican National Committee of packing its debate audiences with donors—a move he claimed violated the loyalty pledge he signed in September vowing to run as a Republican and support the party's eventual nominee. "I signed a pledge, but it's a double-edged pledge," Trump says at a luncheon in Mount Pleasant, South Carolina. "The pledge isn't being honored by the RNC." But Trump saved much of his criticism for Cruz. "If he doesn't take down his false ads and retract his lies," Trump says in a statement bashing Cruz, he will immediately file a lawsuit challenging Cruz's eligibility to serve as president. (More Donald Trump 2016 stories.)