GOP presidential candidates sparred over how to stop ISIS, in the first debate since attacks in San Bernardino 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.

Donald 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 GOP 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.

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 real estate mogul directly, and if he had been as forceful several months ago as he was last night, his campaign might be in much better shape.

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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, Marco 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 Donald 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.

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 Marco Rubio, taking aim at Ted Cruz, who had voted to curtail government surveillance powers.

Another of Donald Trump’s proposals – “closing that internet up” to stop ISIS 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 program; on several occasions the audience’s cheers and jeers forced a pause in the candidates’ conversation.

At one point, a heckler interrupted Donald Trump with inaudible comments.

Donald Trump loomed large over the so-called undercard debate, with the four candidates split over the efficacy of his proposed ban.

Senator Lindsey Graham apologized to US-allied Muslim leaders saying: “I am sorry. He does not represent us.”

Democrats debate on December 19, 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 February 1^st and will last for months.

Each party will formally nominate their candidate over the summer, with Hillary Clinton the favorite to win the Democratic nomination.

Americans will go to the polls in November 2016, and the newly elected president will assume office in late January of 2017.