Republican frontrunner Donald Trump and the party's one-time expected nominee, former Florida governor Jeb Bush, clashed in the party's fifth presidential debate over the businessman's plan to bar Muslims.
National security and the response to terrorism dominated the debate, the first since the attacks on Paris and San Bernardino, coming on a day when a threat closed more than 1,000 schools in Los Angeles.
Tetchy exchanges between Mr Trump and Mr Bush and among the three freshman senators in the race, Marco Rubio, Ted Cruz and Rand Paul, dominated the final debate of 2015, hosted by CNN in Las Vegas.
The anticipated face-off between Mr Trump and Mr Cruz, the conservative alternate to the brash businessman, failed to materialise, despite the Texas senator's recent surge into second place and overtaking Mr Trump in some polls in the first-nominating state Iowa.
The more personal exchanges instead came between the man seeking to become the third President Bush and the reality TV star who has stunned the establishment with his brash, outsider campaign.
The former two-time governor attempted to breathe life into a flagging campaign, looking to make an impression for the first time on a debate stage.
Mr Bush attacked Mr Trump’s proposal to bar Muslims from entering the US, attempting to undermine his credibility as commander-in-chief at a time when terrorism has in recent weeks become the number-one concern for the American people.
The plan would weaken efforts to build a strong coalition among Muslim states to defeat Islamic State militants, said Mr Bush, before attacking the businessman personally over his propensity to insult.
“Donald, you know, is great at the one-liners but he’s a chaos candidate and he’d be a chaos president. He would not be the commander-in-chief we need to keep our country safe,” he said.
Mr Trump shot back, saying that Mr Bush called him “unhinged” over his Muslim plan because he has fallen behind in the polls.
“He said that very simply because he has failed in this campaign. It’s been a total disaster. Nobody cares,” he said of his rival.
The property tycoon had to defend his radical plan from the outset, responding to his first question on whether he wanted to isolate the US.
“We are not talking about isolation. We’re talking about security. We’re not talking about religion. We’re talking about security,” he said.
The plan was scarcely mentioned during the three-hour debate, despite the condemnation he attracted at home and abroad.
The two sparred again when Mr Trump challenged the former two-term governor on his remarks that Mexican immigrants crossed the border as an act of love, while Mr Bush criticised his stance on Isis.
“I know you’re trying to build up your energy, Jeb, but’s it not working very well,” said the businessman, repeating his regular “low energy” jab at the former governor.
“You’re not going to be able to insult your way to the presidency,” Mr Bush told the businessman, to cheers from the crowd at The Venetian hotel and casino on the Las Vegas strip.
Mr Trump wasn't alone in being a target of attacks. Mr Rubio, the Florida senator who is running third in the polls, was challenged on national security and immigration by his two Senate colleagues.
Mr Cruz, who had a less assured debate than previous, accused Mr Rubio of waging "false" attacks on his position on the government's phone surveillance programme. He was joined by Mr Paul, a long-time critic of the National Security Agency, in attacking Mr Rubio.
The Kentucky senator waged another assault on the Florida senator claiming that his support of a Senate comprehensive immigration bill made him the “weakest of all the candidates on immigration.
“He is the one for an open border that is leaving us defenceless,” said Mr Paul, attempting to advance from his ninth-place position.
Mr Rubio didn’t have the runaway performance he enjoyed in previous debates but he showed himself to be a capable debater, knowledgable on foreign policy and well able to defend himself.
In a big moment for the party on the night, Mr Trump ruled out running as an independent, a nightmare scenario for Republicans that would inevitably split voters and hand the Democrats, most likely Hillary Clinton, victory in the November 2016 run-off election.
“I’ll be honest, I really am,” said Mr Trump when asked if he would abide by a pledge not to stand as an independent, despite wavering on the issue in response to sharp Republican criticism of his Muslim plan.
Mr Trump said that he had “gained great respect for the Republican leadership” and that he was “totally committed” to the party.
The final Republican debate of 2015 left few signs of how the large field might be trimmed with just seven weeks until the first voters start picking nominees for each of the parties in Iowa on February 1st.