The Donald Trump-Ted Cruz Showdown and 5 Other Things to Watch for in the GOP Presidential Debate

— -- The final Republican debate of 2015 is on Tuesday -- and momentum matters now more than ever.

With less than 50 days left before the Iowa caucuses, Donald Trump is climbing to new heights in national polling, hitting a new high of 41 percent support and a broad 27-point lead, in a Monmouth University poll released today.

But in the crucial first-in-the-nation caucus state of Iowa, Trump is locked in a close battle with Sen. Ted Cruz, who is stealing away support of evangelicals, very conservative voters and tea party supporters.

A Quinnipiac University poll today shows the two in a virtual dead heat, with Trump garnering 28 percent support and Cruz with 27 percent. All eyes will turn to the two Iowa front-runners in the center of the stage Tuesday night and everyone is asking one question: Will this be the moment that the gloves finally come off?

The closest the Texas senator has gotten to an attack came from behind closed doors, according to an audio recording provided by The New York Times. "Who am I comfortable having their finger on the button? Now, that's a question of strength, but that's also a question of judgment and I think that is a challenging question for both of them," Cruz said.

Cruz immediately dialed the comments back, failing to attack the real estate mogul at all over the last several days. And for his part, Trump has already called Cruz "a maniac" on Fox News for his performance in the Senate, but Cruz laughed it off.

Trump and Cruz won’t be the only two candidates to watch in Las Vegas on Tuesday. The undercard debate -- featuring Mike Huckabee, George Pataki, Lindsey Graham and Rick Santorum -- is slated for 6 p.m. The top nine candidates will face off at 8:30 p.m.

Here are five other candidates to keep your eye on during Tuesday's Republican debate.

1. Ben Carson has been plummeting in recent national and Iowa polls while voters question his foreign policy credentials in the wake of the terror attacks in Paris and San Bernardino. His favorability numbers have been slipping. He’s tried to combat his slide in support with a trip overseas to visit Syrian refugees and unveiling another foreign trip to Nigeria, Kenya and Zambia. But now that national security and terrorism are dominating the news cycle and these issues top the lists of most important issue for many Republican voters, the neurosurgeon will need to convince voters of his foreign affairs chops to stop his downward spiral.

2. Jeb Bush hasn’t made up any ground since the last Republican debate in early November. The establishment favorite clocks in at just 3 percent support in the most recent national Monmouth University poll. This may not be an ultimate moment of truth for the former Florida governor, but time is running short. For anxious donors and supporters eager to fall in line behind an establishment candidate who they believe can topple Cruz and Trump from atop the Republican field, the clock is ticking. If Bush doesn’t have a standout performance -- one that convinces voters and donors to consolidate around him as the best chance to win the nomination -- it’s likely his backing will sink even lower.

3. Marco Rubio has also remained even in national and early state polls, despite strong reviews on his performance in Milwaukee in early November. According to the prediction market Pivit, Rubio’s odds of winning the GOP nomination have dropped from 49 percent in late November to just 33 percent now following the rise of Ted Cruz, another young Hispanic senator. With other candidates like Carson and Cruz clearly moving one way or another, Rubio has stayed stagnant. With some Republicans looking for an “anti-Trump” candidate, watch for Rubio to focus fire more on Cruz than on Trump, as he tries to distinguish himself once again as the clear viable alternative to Trump in the GOP field.

4. Chris Christie is making a big return to the main stage after getting bumped down to the undercard debate in Milwaukee a month ago. While the New Jersey governor shows little momentum on a national stage, he’s gaining some traction and key endorsements in the first-in-the-nation primary state of New Hampshire. One standout moment in the debate could catapult his campaign into the top tier in New Hampshire, giving front-runner Donald Trump strong competition in both early states and paving the way for Christie to earn broader national support. Ohio Gov. John Kasich, also trying to compete in New Hampshire, faces an uphill climb with limited national exposure.

5. Rand Paul and Carly Fiorina barely scraped by to reach the main stage debate, and time is running short for them to make a move. Hot off strong debate performances earlier this summer, Fiorina rocketed to the top-tier both nationally and in New Hampshire, but she’s since fallen to single digits, most recently clocking in at just 2 percent in the latest national Monmouth University poll. And Rand Paul -- who was allowed into the main stage debate despite averaging 0.33 percent too low in Iowa polling to meet CNN’s threshold -- is also desperately in need of a moment to steal momentum away from Ted Cruz. Both candidates are facing uphill climbs to successful showings in early state contests.