In US: Republican presidential race erupts into four-way battle

Cruz dared Trump to sue him over the campaign ad, which featured a 1999 video clip of Trump, a billionaire businessman and former reality TV star, saying he was "very pro-choice" on abortion.

The Republican race for the party's 2016 presidential nomination erupted into a four-candidate crossfire on Wednesday over who has the proper experience and is the most conservative, days before South Carolina voters put their stamp on the campaign.

In TV interviews and campaign events, front-runner Donald Trump threatened to sue U.S. Senator Ted Cruz of Texas over a negative TV ad, while U.S. Senator Marco Rubio of Florida accused Cruz of lying about his record, and former Florida Governor Jeb Bush questioned Rubio's experience to serve as president.

Amid the squabbling, South Carolina Governor Nikki Haley endorsed Rubio as the Republicans' best hope for winning the White House on Nov. 8, a big boost to Rubio and a blow to Bush, who had lobbied hard for her to pick him.

Bush had a tough day, learning of Haley's endorsement right before a town hall meeting in Summerville, South Carolina, where four members of the audience second-guessed his campaign strategy and offered tips on how to have a greater impact in the state that holds its Republican primary on Saturday.

Much of the debate took place over the television airwaves, as Trump lobbed charges at his rivals on an MSNBC town hall, and Rubio and Cruz battled in back-to-back appearances at a CNN town hall in Greenville, South Carolina.

Trump fumed about a TV ad put out by the Cruz campaign that brought up Trump's past position in support of a woman's right to an abortion. Trump says he has evolved into a conservative and is now against abortion.

"You look at a guy like Ted Cruz, he's a nasty guy," Trump said. "He doesn't have one Republican endorsement from the Senate and he works with the Senate. Think of it. Hard to believe."

Cruz dared Trump to sue him over the campaign ad, which featured a 1999 video clip of Trump, a billionaire businessman and former reality TV star, saying he was "very pro-choice" on abortion.

Cruz said Trump sent his campaign a "cease and desist letter" demanding it drop the ad. At the CNN town hall, he said he laughed out loud when he got the letter threatening what he called a "frivolous lawsuit."

"In any defamation case, truth is a complete defense," Cruz said.