National Security, Muslim Immigration, and Islamic State: What to Watch for in Tonight's Republican Debate

It's been five weeks since the last Republican primary debate, and candidates are flying to Las Vegas tonight to do it all over again for the fifth and last time of 2015. They'll have plenty to discuss.

As in previous debates, the top candidates will line up according to poll numbers, starting with reality television star and front-runner Donald Trump at the center, and retired neurosurgeon Ben Carson and Texas Senator Ted Cruz on either side of him. The six remaining participants fanning out in descending order of poll numbers will be Florida Senator Marco Rubio, former Florida Governor Jeb Bush, businesswoman Carly Fiorina, New Jersey Governor Chris Christie, Ohio Governor John Kasich, and Kentucky Senator Rand Paul.

There will also be an earlier debate at 6pm for the lower-polling candidates, featuring former Arkansas Governor Mike Huckabee, former Pennsylvania Senator Rick Santorum, South Carolina Senator Lindsey Graham and former New York Governor George Pataki.

Although the debate's format is basically the same, a lot has changed since the last debate. Trump has kept himself busy over the past month by making things up about religious and racial minority groups, including tweeting false statistics about black people killing whites and insisting Muslims celebrated during the aftermath of 9/11. More importantly, this is the first debate since the Paris terror attacks and San Bernardino mass shooting occurred, which has brought the topic of national security front and center in the election. It will also be the main focus tonight.

Related: Jeb Bush Is Cool With Syrian Refugees Who Can 'Prove' They Are Christian

CNN moderator Wolf Blitzer said candidates should expect to face questions about terrorism and the Islamic State, which is "the number one issue facing the American people right now." According to a recent Gallup poll, the majority of Americans name terrorism as their main concern, the highest number in a decade.

All of the candidates will try to one-up each other to show just how tough on terrorism they are, but arguably no one has been more belligerent and controversial on the subject than Trump. His statement last week that he'd like to ban all Muslims from coming into the US provoked resounding criticism from his fellow candidates and the US government alike, with many pointing out that his "plan" was unconstitutional and a threat to national security. Tonight could be an opportunity for the other candidates to go after Trump for his proposal face to face, or at the very least for the moderators to question him about it.

But much like attacking a caged animal, it's anyone's guess how well that will go. Trump's outspoken positions on national security have proved to be popular among Republican voters, even if the majority of the country opposes it. Nearly 60 percent of Republicans say they support Trump's proposal to ban Muslims from the US and fully half of Republicans say they trust him the most on the issue of terrorism, according to a new Washington Post/ABC News poll. Any of the Republican candidates who dare attack Trump on national security will be doing so at their own risk.

The makeup of the Republican field has also shifted considerably since the last debate. Ben Carson has fallen from second to fourth place, with Sunday's Fox News poll showing him polling at 10 percent in Iowa.  Ted Cruz has taken second place with a slew of recent polls showing him alternatively tied or beating Trump for the first time. A Des Moines Register/Bloomberg Politics poll on Saturday showed Cruz beating Trump by a whopping ten points in Iowa. Fox News' Sunday poll also shows Cruz leading Trump 28 - 26 among likely Iowa caucus goers.

To no one's surprise, Trump wasted zero time going after Cruz for having the sheer audacity of competing with him. Trump called the Texas Senator "a bit of a maniac" during an appearance on Fox News Sunday.

I was disappointed that Ted Cruz would speak behind my back, get caught, and then deny it. Well, welcome to the wonderful world of politics!

— Donald J. Trump (@realDonaldTrump)December 13, 2015

Cruz, meanwhile, responded to Trump with a Flashdance reference.

In honor of my friend — Ted Cruz (@tedcruz)December 13, 2015

But aside from the occasional Twitter mentions, Cruz and Trump have pretty much avoided sparring with each other up until now. It remains to be seen if the two firebrand outsiders will turn on each other tonight, considering that they are competing for the same ultra-conservative, anti-establishment voters.

Then there is Marco Rubio — the sweet-faced, establishment ying to Cruz and Trump's outsider yang. The junior Florida Senator, who became the Republican establishment's next best thing after it became clear no one really likes Jeb! Bush after all, is entering the debate in third place. Rubio has been plodding along in the race just fine, but like everyone else it seems these days, struggling to get himself heard over the din of Donald Trump.

"Rubio has to go on the attack," says Ben Domenech, a conservative pundit and columnist, "he needs to make a mark and show he can deliver a punch."

But considering no one seems to want to take on Trump directly for fear of losing an eye, Rubio could turn on Cruz tonight — which has quietly become the defining fight of this primary. Rubio has criticized Cruz for his stance on national security, calling him an "isolationist" on Sunday's Meet the Press for voting against mass surveillance and defense spending bills. Cruz, meanwhile, can hit Rubio back for being weak on immigration by supporting a pathway to citizenship for undocumented workers.

Related: In a Rare Work Appearance, Marco Rubio Says Government Employees Who Don't Do Their Jobs Should Be Fired

The main priority of the rest of the stage will mostly be to get voters to pay attention to them. Carly Fiorina, whose past debate performances briefly made her a star, has since faded. Rand Paul almost didn't qualify for tonight's debate and might try to use his tough national security positions as the only leverage he has against Rubio and Cruz in order to stand out on the stage. The Paul campaign hinted as much this week, by releasing an ad on Monday, titled "Learn from Mistakes," which went after Rubio and Cruz's refugee plans as being too soft.

There are two more Republican debates before the Iowa caucuses begin in about six weeks. Tonight's debate could be the last one with two crowded stages, since many of the lower polling candidates can be expected to begin dropping out as voting in the early primary states get closer. But judging how the race has been going so far, any prediction of candidates' behavior is probably a risky one.

Follow Olivia Becker on Twitter: @obecker928