Billionaire businessman Donald Trump refused to rule out running for the White House as an independent, during the first TV debate for Republican candidates.
The event in Cleveland ended speculation that the controversial tycoon would moderate his harsh denouncement of America’s politicians.
If Mr Trump runs as an independent he would probably split the Republican vote, making it more likely that Democrat frontrunner Hillary Clinton would win, giving her party a third straight term in the White House.
Mr Trump was at centre stage because he has run up a considerable polling lead among the 17 Republicans running for the nomination.
He was the only one of 10 candidates in the main debate to raise his hand when the Fox News hosts asked who would not pledge to support the eventual party nominee.
“I will not make the pledge,” he said.
Mr Trump brushed aside questions about his public denigration of women and said he had done nothing but used American laws when four of his companies took bankruptcy.
Mr Trump's refusal to take the pledge enraged Kentucky senator Rand Paul, who said Mr Trump was "already hedging his bets because he's used to buying politicians".
Through the remainder of the debate, the candidates made little news, choosing instead to use their time in the two-hour session to repeat already well-known positions.
But Mr Trump stood out for his willingness to stand behind many of his past statements that many expected would be ruinous to his campaign. Instead he has risen quickly in the polls to become the front-runner.
Fifteen months from the election, Mr Trump remains a longshot candidate to replace President Barack Obama. Only 10 of 17 Republican candidates were invited to participate in the main event, with the remaining seven relegated to a pre-debate forum.
It was a key test for Mr Trump, whose unpredictable style and unformed policy positions mean he does not fit neatly into any single wing of the Republican Party.
That appears to be a draw to some Republicans frustrated with Washington and career politicians,.
But others fear his eccentricities and outlandish comments – whether about Mexican immigrants being “criminals” and “rapists” or his questioning of the war record of Senator John McCain – will taint the American public’s view of the party.
Standing to Mr Trump’s left on the debate stage was former Florida governor Jeb Bush, a favourite of the wealthy donors and business leaders that populate the establishment wing of the Republican Party.
But Mr Bush, the son and brother of two former US presidents, has struggled to separate himself from the rest of the field and he faces questions about whether his nomination would mark a return to the past.
Immigration and counterterrorism dominated the early stages of the debate, two issues that highlight the deep divisions within the Republican Party.
Mr Bush, whose wife was born in Mexico, defended his call for a path to legal status for some of the people living in the US illegally. It is an unpopular position among some Republican voters who equate legal status with amnesty.
“The great majority of people coming here have no other option,” Mr Bush said.
Mr Paul and New Jersey governor Chris Christie engaged in a heated exchange over the USA Patriot Act and laws giving government access to Americans’ phone records.
Mr Christie said he was the only person on the stage to file applications under the Patriot Act and gone before secretive courts for authority.
Mr Paul, a staunch opponent of the surveillance programmes, said he wanted to collect more records from terrorists, not law-abiding Americans.
The remaining seven candidates who did not make the cut for the main debate were relegated to a pre-debate forum, a low-key event in a largely empty arena.
The event was the first of six Republican Party-sanctioned debates scheduled before primary voting begins in February.