Billionaire Donald Trump has refused to rule out an independent run for the White House should he fail to win the Republican nomination, igniting the first major debate of the 2016 presidential 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 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 a fiery beginning to Republican's quest to choose candidate for the 2016 presidential race, six months ahead of the start of primary elections.
Trump's unapologetic, off-script style offends some, but has set him apart from a packed field 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.
Candidates sought to make an impression on voters - and many aimed at President Barack Obama, Democratic frontrunner Hillary 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.
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 favourite 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 on Thursday as the debate began.
The candidates in Thursday's debate were: Trump, Bush, Walker, Huckabee, Paul, Kasich, neurosurgeon Ben Carson, Senator Marco Rubio, Senator Ted Cruz and Governor Chris Christie of New Jersey.