Trump threatens to sue Cruz unless his rival apologises

Republican presidential candidate Donald Trump has offered 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 apologises 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 pivotal Republican primary, the 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, former Florida Gov. 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 yesterday 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 said at a luncheon in Mount Pleasant, South Carolina. "The pledge isn't being honored by the RNC."

But Trump saved the bulk of his criticism for Cruz. "If he doesn't take down his false ads and retract his lies," Trump said in a statement bashing Cruz, 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 many legal experts have said he meets that test.

"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.