WASHINGTON — Tuesday night marks the final Republican primary debate of 2015, and the last chance this calendar year for the candidates to make their pitch to millions of voters before the race heads into 2016.

Republican strategists expect the debate to focus heavily on national security, following the mass shooting in San Bernardino, Calif., carried out by a couple who were reportedly radicalized by the Islamic State (ISIS) and the recent terrorist attacks in Paris.

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The primetime event will feature businessman Donald Trump, retired neurosurgeon Ben Carson, Texas Sen. Ted Cruz, Florida Sen. Marco Rubio, former Florida Gov. Jeb Bush, New Jersey Gov. Chris Christie, former HP chief executive Carly Fiorina, Ohio Gov. John Kasich and Kentucky Sen. Rand Paul.

CNN has rearranged the lineup for Tuesday's debate. Christie in, Huckabee out. #GOPDebate pic.twitter.com/0gI7nv5Psf — Reagan Fan (@Im_ur_hklbry) December 13, 2015

Four more Republican candidates, whose polling did not qualify them for a spot on the main stage, will participate in a so-called "undercard debate." They are former Arkansas Gov. Mike Huckabee, former Pennsylvania Sen. Rick Santorum, South Carolina Sen. Lindsey Graham and former New York Gov. George Pataki.

Here's what to watch:

How hard does Trump go after Cruz

After months of peacefully coexisting in the GOP primary field, Trump has now shifted his attacks to Cruz — whose rise in Iowa threatens Trump's quest for the nomination.

Following the release of a Des Moines Register-Bloomberg poll that showed Cruz skyrocketing to first place in Iowa, Trump unleashed a verbal assault on Cruz's character.

Trump called Cruz "a little bit of a maniac" for his performance in the Senate in an interview with Fox News Sunday morning.

He later attacked Cruz on Twitter, calling the Texas senator out for comments Cruz reportedly made at a private fundraiser, questioning whether Trump had the "judgment" to be president.

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

Will Trump and Cruz finally take the gloves off?

Can Cruz deflect Trump's fire?

So far, Cruz has avoided being the focus of Trump's attacks on the debate stage. But if Trump does take Cruz on, it's Cruz's response that will be the story of the night.

Until this point, Cruz has declined to criticize Trump, likely waiting to gobble up Trump supporters in the event of a possible Trump implosion. And Republican strategists say that will make deflecting Trump's ambush a challenge for Cruz.

"Here’s the problem for Ted Cruz: For months he has been down fetching his shine box every time Trump says a word. Now, to ... say, 'Donald Trump, you’re wrong, you're unqualified to be president, your finger should never be on the nuclear button and I denounce you as scourge,' he cannot do it," said Rick Wilson, a Florida-based Republican strategist. "Ted Cruz has been feeding the alligator in hopes that the alligator eats him last, but the alligator is now hungry and it’s going after him."

Cruz gave a preview for how he could respond in a tweet Sunday night, after Trump called him a "maniac."

How will the field respond to the Muslim immigration plan?

Trump's plan to temporarily ban Muslims from immigrating to the United States has been repudiated by almost every one of his Republican rivals, as well as leading Republicans including House Speaker Paul Ryan.

The moderators of Tuesday night's debate will almost certainly ask every candidate on stage to respond to the policy again. Will any of the candidates change their response standing side-by-side with Trump on stage?

Can Christie build on his budding momentum?

An endorsement from the New Hampshire Union Leader, an influential conservative newspaper in the Granite State, reinvigorated a flailing Christie campaign in November.

Since then, Christie has seen his poll numbers climb in New Hampshire, launching him back onto the primetime stage Tuesday — after he was relegated to the undercard stage last month.

Republican strategists say they are waiting to see if a debate focused on national security provides even more of an opening for the New Jersey governor to build on that momentum. He often touts his record as U.S. attorney of New Jersey, where he prosecuted terrorists following the terrorist attacks on Sept. 11, 2001.

Christie though also runs the risk of being drowned out by potential fireworks between Trump and Cruz.

Will anyone in the lower tier gain traction?

With just 47 days to go until the Iowa caucuses, just four candidates have broken into the double-digit polling average, according to RealClearPolitics: Trump, Cruz, Rubio and Carson. The rest of the candidates are still hovering in the low single digits — a troubling sign for their campaigns as primary contests rapidly approach.

For Bush, who despite tens of millions in television ad spending has seen his poll numbers decline, a strong debate performance is crucial to convincing donors and voters that he is still a viable candidate.

Even so, multiple Republican strategists doubt a good debate will be enough to catapult candidates in the single-digit polling average — especially those in the undercard debate — to top-tier status.

"I think the rest of the field will look like condemned men/women waiting for the electric chair," said Bob Kish, an Ohio-based GOP ad maker.