By David Lightman, McClatchy Washington Bureau (TNS)

LAS VEGAS — Ted Cruz enters Tuesday’s Republican presidential debate poised to become the Republican to beat.

The senator from Texas has surged into the top spot in Iowa polls, buoyed by endorsements from leading evangelical voices. Real estate mogul Donald Trump remains the national front-runner, but it is Iowa voters who will shape the race first when they caucus in seven weeks. Cruz also gets a boost from a huge campaign treasury.

All that makes the final Republican debate of 2015 crucial, as it will leave impressions that are likely to linger into the new year. And it signals a change for Cruz, as he now confronts the sort of intense scrutiny that faces newly minted national political stars.

Trump enters the debate already swinging away. He and Cruz had been careful not to criticize each other, aware they were appealing to the same fed-up voters who get information and reassurance from conservative talk radio and social media. But once Cruz started surging in polls over recent days, Trump started hitting. On Sunday, he told Fox News that Cruz was “a little bit of a maniac” in the Senate.

The main debate at Las Vegas’ Venetian Hotel begins at 9 p.m. EST and will feature nine candidates. Four others will vie in an earlier debate.

Viewers are likely to see one new wrinkle Tuesday: Foreign policy and domestic security are likely to be more prominent topics. Since the last debate Nov. 10, terrorist attacks rocked Paris and mass shootings at a Colorado Planned Parenthood clinic and a San Bernardino holiday party have dominated the news.

Here are four things to watch for:

On Cruz control

Cruz has jumped over retired neurosurgeon Ben Carson as the alternative to Trump for frustrated, angry conservatives. Now he’s got to maintain his exasperation with Washington while still appealing to Republicans uncomfortable with Trump’s temperament.

The bigger challenge: Opponents will try to stagger Cruz by noting his Senate colleagues’ disdain for him and recalling his no-compromise tactics as well as his past willingness to shut down the government. He’s known largely in the Senate as a flame-thrower, not a doer.

Trump vs. Muslims

This will be the first face-to-face clash since Trump called for a ban on all Muslims entering the U.S. Top Republicans criticized it. How will they do it onstage? What will Trump say? Will Cruz find a way to disagree while praising Trump personally, as he’s already done?

“He’s like a shock jock. He knows how to get ratings,” said Rep. Mario Diaz-Balart (R-FL). Trump is unlikely to apologize and inclined to offend.

History says that as people get more serious about picking a president, particularly one who could lead a nation at war, they’ll weigh the importance of Trump’s temperament. Does he seem presidential? Is he too eager to make headlines, instead of offering serious talk about policy?

Where’s the mainstream?

A sizable chunk of the Republican electorate wants someone other than Trump or Cruz and might accept a more conciliatory candidate with a history of working with Democrats. So far, no one has emerged as a favorite.

Chris Christie, the governor of New Jersey and a former federal prosecutor, has had a boost as national security has become a more prominent issue. He’ll be on the main stage Tuesday after being relegated to the undercard last month. John Kasich, the brash governor of Ohio, has tried to become Trump’s biggest nemesis, but it hasn’t worked. Jeb Bush, the former Florida governor, remains stuck far down in most polls. Each badly needs a breakout moment Tuesday.

Marco Rubio

The pundits have been waiting for the U.S. senator from Florida to emerge as the choice of mainstream voters. For awhile last month, as Trump maintained his lead, establishment types began rallying around Rubio, but the momentum wasn’t there and he didn’t become the clear alternative.

The potential remains, and his command of foreign policy could vault him into that position. He needs to show movement soon, or he risks getting lost behind Cruz and possibly one of the more center-right challengers.

(c)2015 McClatchy Washington Bureau. Distributed by Tribune Content Agency, LLC.

U.S. Republican presidential candidate Ted Cruz speaks at a 2nd Amendment Coalition announcement at CrossRoads Shooting Sports in Johnston, Iowa, December 4, 2015. REUTERS/Brian C. Frank