Republican presidential hopefuls sparred over how to stop the so-called Islamic State (IS), in the first debate since attacks in California and Paris.
The national security focus yielded heated exchanges between Senators Ted Cruz and Marco Rubio who clashed on surveillance and immigration policy.
Jeb Bush also sought to revive his struggling campaign by forcefully attacking front-runner Donald Trump.
“You’re not going to be able to insult your way to the presidency,” he said.
Mr Trump was on the defensive early in the debate for his proposed ban on Muslims entering the US, saying, “We are not talking about religion, we are talking about security.”
However the debate quickly expanded to broader issues of foreign policy and national security.
The candidates repeatedly addressed heightened fears of terrorism in the US on the same day an emailed threat shut down Los Angeles’ school system.
Analysis: Anthony Zurcher, BBC News, Las Vegas
The audience was outspoken during the debate
The big question going into this last Republican debate of 2015 was how Donald Trump’s competitors would try to take the front-runner down.
It seems, however, that only Jeb Bush got that memo. He alone among the candidates engaged the New Yorker directly, and if he had been as forceful several months ago as he was on Tuesday night, his campaign might be in much better shape.
Instead, most of the fireworks during the Las Vegas event occurred between the trio of first-term senators – Marco Rubio, Rand Paul and Ted Cruz.
On the issues of national security and immigration, Mr Rubio faced off against his two congressional colleagues in often acrimonious exchanges.
Barely mentioned over the course of an evening that focused on foreign policy was Mr Trump’s call to close the US border to all Muslims.
Given how all the candidates assiduously avoided the subject, one would never have guessed that it was a story that merited global headlines and ignited a firestorm of controversy.
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But the top nine candidates disagreed over the scope of government surveillance and how to end the civil war raging in Syria.
“If terrorists strike again… the first question will be, ‘Why didn’t we know about it and why didn’t we stop it?'” said Mr Rubio, taking aim at Mr Cruz, who had voted to curtail government surveillance powers.
Another of Mr Trump’s proposals – “closing that internet up” to stop IS recruitment – has been hotly debated, with the candidate saying, “I don’t want them using our Internet”.
After defending it, he seemed confused by loud booing from the audience, and replied: “These are people that want to kill us folks.”
It was not the only time that the crowd played a part in the programme; on several occasions the audience’s cheers and jeers forced a pause in the candidates’ conversation.
At one point, a heckler interrupted Mr Trump with inaudible comments.
Image caption Mr Kasich called for the country become more united
Other highlights from the debate included:
- Despite expectations of a confrontation between the top candidates – Senator Cruz and Mr Trump – the men avoided directly criticising each other
- Mr Trump said that he would not pursue a third-party campaign, saying he had “gained great respect” for the party’s leadership
- When asked whether he could order air strikes that could kill civilians or children, Ben Carson pointed to his experience as a paediatric surgeon and having to tell children about brain cancer
- Kentucky Senator Rand Paul stuck to his strong libertarian foreign policy beliefs, despite stark differences with the other candidates
- New Jersey Governor Chris Christie repeatedly stressed his executive experience as a governor and prosecutor and took a swipe at senators like Mr Cruz and Mr Rubio saying they were all talk
- Ohio Governor John Kasich said world leaders would have been better off discussing terrorism than climate change at a recent summit in Paris
- Former Hewlett Packard CEO Carly Fiorina said help from the private sector should be sought to fix an “incompetent” government
The Republican contest – in depth
How to dump Trump – four ways his rivals might beat him at the debate
Muslims on Trump – Muslims in Las Vegas reflect on Trump ban plan
Cruz on the rise – five things to know about Trump’s biggest threat
Clinton’s secret agent? – Conspiracy theorists claim Trump is Hillary Clinton’s plant
Mr Trump loomed large over the so-called undercard debate, with the four candidates split over the efficacy of his proposed ban.
Senator Lindsey Graham apologised to US-allied Muslim leaders saying: “I am sorry. He does not represent us”.
Democrats debate on Saturday night, and both parties will hold debates in January.
The state-by-state primary contests in the presidential election begin in six weeks in Iowa on 1 February and will last for months.
Each party will formally nominate their candidate over the summer, with Hillary Clinton the favourite to win the Democratic nomination.
Americans will finally go to the polls in November, and the newly elected president will assume office in late January of 2017.