Donald Trump refused to rule out an independent White House bid and bristled at questions about his attitudes toward women, leaving his rivals struggling for attention on Thursday during first Republican presidential debate.
The nine other Republicans sharing the stage with Trump turned on one another at times but largely avoided direct challenges to the combative real estate mogul who has rocketed to the top of opinion polls in the 2016 race.
Senator Rand Paul of Kentucky did take on Trump, confronting him after Trump kicked off the debate by refusing to pledge his support for the Republican nominee in the November 2016 election.
"I will not make the pledge at this time," said Trump, who for weeks has said he would not rule out an independent bid that would almost certainly split the Republican vote and boost the chances of victory for the Democratic nominee.
Trump's response drew boos from the crowd and a rebuke from Paul, who said Trump was keeping his options open to support Democratic front-runner Hillary Clinton, a reference to his past friendship with both Clinton and her husband, Bill.
"He's already hedging his bets because he's used to buying politicians of all stripes," Paul said.
Trump's veiled threat to bolt the party if necessary could risk his support among Republican primary voters and stall his momentum, although the billionaire reality television star has repeatedly defied predictions of political doom.
In a show of hands, participants in a Fox News focus group who supported Trump before the debate overwhelmingly indicated they had changed their minds about him, critical of his refusal to commit to backing the eventual Republican nominee.
"I think he took out a gun and shot his campaign in the head," said Florida Republican strategist Rick Wilson.
Watch: How Donald Trump stole the show at Republican debate
No clear winner
No candidate emerged the clear winner from the two-hour debate, leaving the Republican race potentially more unsettled than ever. A record 17 candidates are vying for their party's nomination.
During a combustible performance, Trump fired off insults at Paul, Fox News moderator Megyn Kelly, comedian Rosie O'Donnell, the Mexican government and an assortment of other targets.
He tangled with Kelly when she pressed him about past derogatory comments he had made about women, including calling them "fat pigs," "dogs," and "slobs," Trump dismissed the question as "political correctness."
He accused Kelly of not treating him well, drawing more boos from the audience.
"Honestly Megyn, if you don't like it, I'm sorry. I've been very nice to you although I could probably maybe not be based on the way you have treated me," Trump, whose base of support is overwhelmingly male, said to a mixture of boos and applause.
The debate's feisty tone suited the flamboyant Trump, who has been the center of campaign attention for weeks for his personal attacks on rivals and his scathing comments about US senator John McCain's war record and about Mexican immigrants.
Trump kept it up in the debate, calling the Mexican government "much smarter, much sharper, much more cunning and they send the bad ones over."
The sometimes combative nature of the debate made bystanders at times of more measured rivals such as former Florida governor Jeb Bush, who is No. 2 in the polls, and Wisconsin governor Scott Walker.
Bush called Trump's rhetoric "divisive" and said "we're going to win when we unite people with an optimistic message."
Bush's senior advisers said after the debate they were happy with his mistake-free performance, which focused on him talking up his record as Florida governor.Retired neurosurgeon Ben Carson finished second behind Trump in the number of mentions on Twitter. Trump led with 30%, while Carson was second at 12% and Paul was third with almost 10%.
Fellow Republican 2016 US presidential candidates Dr Ben Carson (L), Wisconsin governor Scott Walker (2nd L) and former Florida governor Jeb Bush (R) laugh as fellow candidate and businessman Donald Trump (2nd R) reacts near the end of the debate after realizing that a slew of criticisms spoken by fellow candidate and former Arkansas governor Mike Huckabee (not pictured) were not aimed at him but at Democratic presidential candidate Hillary Clinton, at the first official Republican presidential candidates debate of the 2016 US presidential campaign in Cleveland, Ohio. (Reuters Photo)
The debate was the first chance for Republican voters to make a side-by-side comparison of the top 10 presidential contenders. Trump's rivals had to decide whether to directly confront him or to try to stay above the fray and look presidential.
Paul, who has lagged in the opinion polls, went on the offensive. In addition to challenging Trump on his potential independent run, Paul accused Trump of not realizing that Republicans opposed a single payer healthcare system. But Trump dismissed his attacks.
"You're having a hard time tonight," Trump said.
New Jersey governor Chris Christie belittled Paul's efforts in the Senate to curtail the government's electronic surveillance system, saying there should be more tools for tracking terrorists.
"When you're sitting in the subcommittee just blowing hot air about this, you can say things like that," Christie said.
Before the main event, seven candidates whose poll ratings did not qualify them for prime time took part in a separate daytime debate. Several challenged Trump's conservative credentials, noting he had changed positions on abortion, healthcare and other issues.
But Carly Fiorina, a former business executive and the only woman in the Republican field, acknowledged Trump had tapped into a broad sense of frustration with Washington.
"Whatever your issue, your cause, the festering problem you hoped would be resolved, the political class has failed you. That's what Donald Trump has tapped into," said Fiorina, who was the runaway choice at more than 80 percent when Fox News asked viewers to tweet who they thought won the first debate.
Shortly after the early debate, social media interest in Fiorina surpassed interest in Trump, according to Google analytics. The next Republican debate is on September 16.