Watch for Trump-Cruz smackdown in this debate

Ted Cruz gains ground ahead of 5th GOP debate

Republicans head back to the debating stage Tuesday night in Las Vegas with a new dynamic at place in the race: Donald Trump vs. Ted Cruz. Trump continues to hold solid leads in national polls but Cruz now has the edge in Iowa, which holds it caucuses in just over a month.

In an ordinary campaign, a front-runner like Trump could ignore a close Iowa loss and move on to friendlier terrain in New Hampshire and beyond. But this is not an ordinary campaign and Trump is not an ordinary candidate.

The braggadocio billionaire's entire persona is based on always winning, and he has boasted that he would absolutely win Iowa. If he doesn't, the entire dynamic around Trump could change. He would have the stink of the "loser" — Trump's favorite dismissive word for critics — on him and it's not clear he could recover.

GOP presidential candidates Senator Ted Cruz and Donald Trump.
Pete Marovich | Bloomberg | Getty Images

The numbers in New Hampshire could shift rapidly after Iowa, as could the other early states of South Carolina and Nevada. National GOP polls at this point remain an interesting data-point about the mood of the party but little more.

So Trump, who has already labeled Cruz a little bit of a "maniac," will likely go on the offense against the Texas senator in the CNN debate. But Cruz, who has assiduously courted Trump supporters, may not take the bait. So far, every time Trump has come after him, Cruz has laughed it off and spoken of The Donald as his "good friend."

While doing this, Cruz has also spoken in private of Trump as lacking the temperament to be president. The Texas conservative firebrand appears content to praise Trump publicly while sowing doubts about him in private. Don't expect that to change in Las Vegas on Tuesday night, though Cruz will have to respond if Trump hits him on specifics.

Among the other candidates, pressure is again on former Florida Gov. Jeb Bush, who still has not moved at all in the polls. Somehow he needs to show some fire and go on a sustained attack against Trump and others in the field. The question is whether Bush has it in him to do this. He is not a natural attack dog, preferring policy discussion to trading barbs and insults. This reporter spoke with an unaffiliated GOP operative in New Hampshire last week who said he saw no indication that Bush would be able to transition to the kind of bare-knuckled approach the 2016 campaign seems to require.

"You need to be able to know your spots and then stick the knife in repeatedly when you have the chance," the operative said. "Jeb doesn't really want to do that."

For the rest of the candidates, the drill remains about the same. Florida Sen. Marco Rubio is generally very strong in debates, though he could stand to seem a little less polished and scripted. His path to the nomination remains being the center-right, establishment favorite and alternative to Cruz or Trump. Expect Rubio to train fire at Cruz over the Texas senator's votes on privacy and national security. Rubio thinks Cruz is vulnerable on the issue after San Bernardino and Paris and he may be right.

President Barack Obama addresses the nation from the Oval Office at the White House in Washington, DC, on December 6, 2015.
Obama address sets stage for 2016 election campaign
Republican presidential candidate Donald Trump (L) and Ben Carson (C) looks on as U.S. Sen. Ted Cruz (R-TX) speaks during the Republican Presidential Debate sponsored by Fox Business and the Wall Street Journal at the Milwaukee Theatre on November 10, 2015 in Milwaukee, Wisconsin.
As Cruz rises in US presidential polls, Trump calls him 'maniac'

New Jersey Gov. Chris Christie has an opportunity to build on his recent rise in New Hampshire by continuing to present himself as electable and expert in issues of national security. In particular, Christie is likely to go after Trump as a pure bluster candidate whose plan to ban all foreign Muslims from entering the country is not worthy of the GOP. If anyone is likely to punch hard and often at Trump, it's Christie.

Ben Carson, in free fall in the polls of late, probably has no path back to the top but he could at least stop the bleeding with a strong performance. For Carson, the trouble is he generally disappears in debates and often seems lost on policy issues. The rest of the candidates remain largely irrelevant. There will be many interesting subplots in the CNN debate. But the most interesting to watch will be the Cruz vs. Trump show.

—Ben White is Politico's chief economic correspondent and a CNBC contributor. He also authors the daily tip sheet Politico Morning Money [politico.com/morningmoney]. Follow him on Twitter @morningmoneyben.