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

Tuesday's Republican debate could be a crucial test for both candidates.

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

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.

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.

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.