By Steve Holland and Emily Flitter
CHARLESTON, S.C./SUMTER, S.C. (Reuters) - 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 Senator Ted Cruz of Texas over a negative TV ad, while 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, a boost to Rubio and a blow to Bush, who had lobbied hard for her to pick him.
The endorsement gave Rubio, 44, a valuable ally to try to sway voters ahead of Saturday's South Carolina Republican primary, the third contest after Iowa and New Hampshire to pick a party nominee for the Nov. 8 election to succeed Democratic President Barack Obama.
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.
Much of the debate took place over the television airwaves, as Trump, 69, a billionaire businessman and former reality TV star, lobbed charges at his rivals at 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 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.
Questioners at the town hall pressed Cruz, 45, on charges leveled against him by Trump that he may not be eligible to be president because he was born in Canada, and that none of his U.S. Senate colleagues liked him.
Cruz said he met the constitutional requirement that a president must be a "natural born citizen" because his mother was an American.
As for his Senate colleagues, Cruz said some may not like him because he serves the interests of his constituents, not fellow senators.
The latest national Reuters/Ipsos poll showed Trump taking a more than 20-point lead over Cruz in the Republican race.
Trump had 40 percent support in the poll conducted from Saturday to Wednesday, compared with 17 percent for Cruz, 11 percent for Rubio, 10 percent for retired neurosurgeon Ben Carson, and 8 percent for Bush.
The results contrasted with those of an NBC News/Wall Street Journal national poll that put Cruz ahead of Trump among Republicans by 28 percent to 26 percent.
Rubio, hoping to overtake Cruz for second place in South Carolina, where Trump leads in the polls, accused Cruz of spreading falsehoods with an ad attacking Rubio over his position on immigration.
Rubio backed a 2013 Senate immigration reform effort that collapsed. Cruz says Rubio would have supported "amnesty" for illegal immigrants, which his rival denies.
"If you say something that isn't true and you say it over and over and you know it's not true, it must be a lie," Rubio said.
Bush, running fourth in South Carolina and behind Rubio, questioned Rubio's level of experience after Rubio said Bush did
not have foreign policy experience. Rubio counts his time on the Senate Foreign Relations Committee as valuable experience.
"It's hard for me to be lectured to by a gifted young guy who thinks going to committee hearings means you know something about the world," Bush said in Summerville.
At the event, Bush, 63, the son and brother of former presidents, sounded a note of frustration at frequent criticism of his campaign by TV pundits.
"The pundits have made it all – it's all decided. We don't have to go vote, I guess. It's all finished. I should stop campaigning, maybe, huh? It's all done," he said.
"No," shouted people in the crowd.
(Additional reporting by Chris Kahn in New York; Editing by Peter Cooney)