CLEVELAND - Brash billionaire Donald Trump on Thursday refused to rule out an independent White House run should he fail to win the Republican nomination, taking center stage in the first major debate of the 2016 race.
Tensions between the Republican party's top 10 candidates soared soon after they took the stage in Cleveland, when Trump said he could not promise to back the eventual party nominee - or that he would not mount a solo campaign.
"I will not make the pledge at this time," the improbable party frontrunner said, to loud boos and jeers from the rambunctious crowd.
The remark triggered an angry exchange with Senator Rand Paul, who shouted across the stage that Trump was "already hedging his bets because he's used to buying politicians." It was an ominous beginning to the party's quest to choose a flagbearer for the 2016 race to succeed President Barack Obama, six months ahead of the start of primary elections.
With 17 major Republican candidates in contention, broadcaster Fox News split the debate into two parts, with bottom-tier hopefuls trading barbs in a separate forum ahead of the prime-time event.
Trump's unapologetic, off-script style offends some but has set him apart from a packed field of hopefuls furiously trying to garner the same level of attention.
"Donald Trump's hitting a nerve in this country," Ohio Governor John Kasich said.
"For people who want to just tune him out, they're making a mistake." Former Florida governor Jeb Bush acknowledged that the bar is likely higher for him in 2016, being the son and brother of two presidents. But he insisted once more that he is his own man with his own policies.
"I'm going to have to earn this," Bush said.
He also stood by his earlier remark that immigrants breaking the law to come to the United States did so as an "act of love." "I believe that the great majority of people coming here illegally have no other option. They want to provide for their family," Bush said. "But we need to control our border." The candidates, each looking for a breakout moment, also focused their ire on Hillary Clinton, the Democratic frontrunner and former secretary of state.
"If Hillary is the candidate, which I doubt, that would be a dream come true," said neurosurgeon Ben Carson, the only black candidate in the field.
"She is the epitome of the progressive, the secular progressive movement."
- Clinton, Trump draw fire -
Candidates at both the main event and on the debate undercard sought to make an impression on voters - and many aimed at Obama, Clinton and Trump.
They offered withering attacks on Obama's treatment of the Islamic State group; vowed tougher immigration policy; pledged to toe the conservative line on social issues; and stressed they would shred a nuclear deal with Iran on day one of a Republican presidency.
"Under President Obama and Secretary Clinton, they're working hard to change the American dream into the European nightmare," said Louisiana Governor Bobby Jindal, one of seven candidates on stage for the early forum.
Carly Fiorina, the former Hewlett-Packard chief, accused Trump of cozying up to the Clintons and slammed him for flip-flopping.
"Since he has changed his mind on (immigration) amnesty, on health care and on abortion, I would just ask what are the principles by which he will govern?" asked Fiorina, who appeared sharp and was seen as a top performer in the early event.
- 'A real race' -
"We've got a real race on our hands," Republican National Committee chairman Reince Priebus told Fox News. The Democratic side, he argued, is holding a Clinton "coronation." The gaggle of Republican candidates is hardly a "Who's Who" of well-known political figures of the right.
After Trump and Bush, Wisconsin's Governor Scott Walker and former Arkansas governor turned television host Mike Huckabee are among the few to have drawn much national attention previously.
Immigration is a favorite subject of Trump, who has promised to build a wall between Mexico and the United States to stop illegal immigrants.
He sparked a firestorm and offended many Hispanic voters when he said Mexico was not "sending its best" and said the immigrants were bringing drugs and crime to the US.
"If it weren't for me, you wouldn't even be talking about illegal immigration," Trump said Thursday as the debate began.
"He's saying a lot of the things that everybody thinks and he says them without being embarrassed," said Brad Roller, in a bar in Cleveland after a campaign event for Senator Marco Rubio.
Democrats have stressed that Trump or no Trump, Republicans differ little on the substance.
The candidates in Thursday's main debate were: Trump, Bush, Walker, Huckabee, Carson, Paul, Kasich, Rubio, Senator Ted Cruz and Governor Chris Christie of New Jersey.