Watch CNN’s coverage of the fifth Republican presidential debate live from Las Vegas on Tuesday, December 15. Coverage begins at 6 p.m. ET.

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What happens in Vegas tonight will not stay in Vegas -- it will have significant implications for the destiny of the GOP nomination

This will be the first Republican debate since the attack by a radicalized Muslim couple in San Bernardino killed 14 people

Washington CNN  — 

It’s debate night in Sin City.

Republican presidential candidates are in Las Vegas for their last debate of the year, just seven weeks before the 2016 nominating race gets underway at the Iowa caucuses.

And what unfolds on stage at the Venetian casino will have significant implications for the destiny of the GOP nomination.

Always controversial front-runner Donald Trump is looking to tighten his grip on the lead spot. Texas Sen. Ted Cruz, suddenly ahead of Trump in the Hawkeye state, wants to justify the hopes of those who see him as the most authentic conservative in the race. And the GOP establishment is increasingly desperate for someone – perhaps Florida Sen. Marco Rubio – to prove he or she can take on Trump and Cruz.

As for the rest of the pack, they are searching for a lucky break.

Here are seven things to watch at tonight’s showdown.

Will Cruz face missiles?

Cruz suddenly finds himself in the top tier of the Republican presidential race – and in the crosshairs of his rivals.

The Texas senator surged to a 10-point lead in the closely watched Des Moines Register/Bloomberg poll of the Iowa caucuses released over the weekend, meaning Cruz can expect plenty of incoming fire on Tuesday night. Cruz has had an easy ride from Trump so far, but the front-runner has started to pointedly question the freshman senator’s readiness for the Oval Office.

“I would say I have far better judgment than Ted, and I think I have really great temperament. It’s a strong temperament,” Trump told CNN’s Jake Tapper on “State of the Union” Sunday.

If Trump does unload, Cruz must parry the attack without alienating Trump’s army of supporters. The Texas senator has being extremely solicitous to the front-runner, hoping to eat into his constituency of anti-Washington voters furious at politics as usual should Trump fade.

Cruz’s strength in attracting evangelical voters and tea party supporters is hardly a surprise, but the Texan is under pressure to show that he can widen his appeal in the party and he needs to show that he is a true rival to the dominant personality of Trump. So Cruz also needs to keep an eye out for the other Cuban-American 40-something on stage, Marco Rubio.

The Florida senator hopes to peel off some of Cruz’s evangelical and social conservative base and build a bridge to establishment supporters who, as former front-runner Jeb Bush continues to fade, are casting around for anyone who is not Trump or Cruz to support.

Rubio has already questioned Cruz on foreign policy, suggesting he is guilty of an “isolationist” approach and criticizing him for voting to restrict the bulk metadata programs used by intelligence agencies – a move that leaves Cruz more exposed following the San Bernardino terror attack.

A showdown between Cruz and Rubio could be a highlight of the primary season, since they are both supremely talented debaters.

Which Donald Trump will show up?

He might be under pressure in Iowa, but Trump is, if anything, cementing his lead elsewhere. In one poll released Monday, by Monmouth University, he was a staggering 27 percentage points ahead of his nearest national challenger, Cruz.

So that gives the Donald a dilemma when he takes his familiar front-runner’s spot at center stage.

There’s one school of thought that says that Trump should play it safe and lock in his front-runner status. After all, it’s only seven weeks until voters start to weigh in. Such an approach may have the virtue of papering over one of Trump’s biggest vulnerabilities, the idea that he’s too bombastic to be commander-in-chief.

At other times during the debate season, he’s been happy to fade into the background. And in interviews over the weekend – following his demand last week for a ban on Muslims entering the United States – Trump seemed a little more mellow than usual.

But Trump also knows that millions of viewers are tuning in to see him put on a show. It’s a dynamic that has been the foundation of his unorthodox presidential campaign for months as he’s identified a rich seam in the Republican Party electorate that feels let down by its leaders and disgusted by what it sees as an epidemic of political correctness.

Will the establishment strike back (starring Marco Rubio)?

It’s time for Rubio to turn potential into poll numbers.

Many establishment Republicans are now looking to the Florida senator as they become increasingly desperate for a candidate to consolidate opposition to Cruz and Trump, whom many elites doubt can win a general election.

“Whoever the establishment person is needs to emerge,” said Matthew Dowd, a former George W. Bush strategist, on CNN’s “Reliable Sources.” “Right now this race looks like it is Donald Trump, Ted Cruz and one other person – who is an establishment person.”

Rubio faces high expectations, mostly of his own making, after turning in a string of quality debate performances. And circumstances may favor him, as he presents himself as the candidate best prepared to serve as commander-in-chief at a time of global uproar and terror threats.

READ: How Donald Trump took the Republican Party by storm

But so far, Rubio does not lead in any of the crucial first four nominating states – leaving him under pressure to chart a path to the nomination.

If Rubio fails to break out at the debate, is there anyone else that could unite the establishment? Bush’s campaign, once seen as a multi-million-dollar juggernaut, is now dormant; the former Florida governor needs theatrics worthy of Las Vegas to turn it around.

New Jersey Gov. Chris Christie, meanwhile, is showing signs of life in New Hampshire following a key local endorsement and climbing poll numbers that have allowed him to re-enter the top-tier GOP debate. He has seized on the aftermath of the terror attacks to present himself as a ruthless potential foe of ISIS. Expect more hawkish national security rhetoric and denunciations of President Barack Obama’s anti-terror record during the debate.

Previous predictions that Ohio Gov. John Kasich could emerge as the establishment champion have fizzled, but he may take on the role of attack dog against Trump after branding the front-runner “dead wrong” for his comments on Muslims.

How strong will the fear factor be?

This will be the first Republican debate since the attack by a radicalized Muslim couple in San Bernardino killed 14 people and brought a revitalized fear of terrorism much closer to home following an ISIS rampage in Paris.

Look for candidates to outdo one another with searing critiques of Obama, who is seen among the conservative base as a feckless and weak leader who fails to understand the Republican orthodoxy that the United States is at war with radical Islam.

“Barack Obama is a bad president because he is an unmitigated socialist who won’t stand up and defend the United States of America,” Cruz said last month, giving the flavor of the kind of rhetoric likely on display Tuesday night.

READ: Terrorism now tops list of voter concerns

But the debate also offers a chance for Trump’s rivals to weigh in on his call last week for Muslims to be temporarily banned from entering the United States.

The proposal was seen by more mainstream Republicans as an example of the kind of overreach that is not just damaging to the party, and will complicate its hopes in November 2016, but which also threatens to tarnish America’s reputation overseas.

Any candidate wishing to take Trump on over the issue will be walking a tightrope, however. New polling by ABC News and The Washington Post released Monday found that 59% of Republicans nationwide supported his plan. The poll also showed that it was opposed by independents and Democrats, encapsulating the Republican Party’s wider vulnerability on Trump’s rhetoric.

Will Carson stop the slide?

No candidate has been hurt more by the campaign’s turn to national security than Ben Carson. The soft-spoken former neurosurgeon briefly threatened to outpace Trump for the outsider vote before dropping more than 10 percentage points in national polls after struggling on foreign policy questions in the wake of ISIS attacks.

Carson is likely to try to seek a recovery by talking about his recent trip to Jordan to assess the plight of Syrian refugees – and to affirm his belief that allowing them into the U.S. poses a risk that terrorists could infiltrate American soil.

Carson may also mention his plans to further his national security knowledge with a trip to Africa over the Christmas and New Year holiday. But unless he demonstrates at the debate that frantic cramming on national security is getting results, his White House hopes may be beyond rescue.

Will the December debate freeze the race?

Conventional wisdom has it that campaigning over Christmas and the New Year rarely moves the needle in presidential races. Voters have more pleasurable diversions than politics over the holiday and the race may freeze in place until everyone gears up to go back to work in January, when Iowa and New Hampshire will be a matter of a few weeks away. That means time is running out for candidates lagging behind to somehow muscle their way into contention, with the debate taking on the aura of a last chance for the likes of Bush, Kasich and businesswoman Carly Fiorina.

And that means that this debate, more than any other clash of the candidates before it, has the potential to have a lasting impact. In fact, it may mark the end of the beginning of the Republican race for president and set the table for the frenzied run-up to the Iowa caucuses in early February.

Will anyone get out of the basement?

Kentucky Sen. Rand Paul made a big deal about getting into the debate and secured a spot only on Sunday after showing viability in Iowa in a Fox News poll.

So far in this campaign, Paul has hardly enhanced his political standing. He’s not managed to harness his father Ron Paul’s libertarian base and has yet to live up to hopes in the GOP that the curly-haired former eye surgeon’s sometimes quirky approach could expand the party’s appeal among younger voters. The isolationist tint of his foreign policy – which once looked a good fit for a war-weary America – now seems more like a liability given the rise of ISIS.

So, for Paul, the debate may be as much about salvaging his political brand ahead of a Senate re-election race next year as engineering an unlikely leap to the top of the presidential pack.

Before the big show, CNN will also host a debate for bottom-of-the-polling-barrel candidates whom most of the political world has forgotten.

Former New York Gov. George Pataki, ex-Pennsylvania Sen. Rick Santorum, former Arkansas Gov. Mike Huckabee and South Carolina Sen. Lindsey Graham will grab what might be one of their final slices of the spotlight in the 2016 race.

They all need a miracle to move out of the basement. But at least one of them, Graham, is having the time of his life running for president. He is the reigning king of the undercard debate and can probably be relied upon for a sprightly performance.