Nine Republican presidential candidates will meet for the party's fifth televised, prime-time debate Tuesday in Las Vegas, with national security concerns heightened and conservative Texas Sen. Ted Cruz emerging to challenge Donald Trump for front-runner status.

Cruz is surging in the Iowa polls, and some in the conservative establishment are beginning to treat him as the reasonable alternative to Trump, though the Texas senator is hardly a favorite of GOP leaders in Washington.

Concern has been growing that Republicans would suffer a landslide loss in 2016 with Trump at the head of the ticket that would pummel the party's down-ballot candidates and, at a minimum, cost control of the Senate.

Yet Trump has been lethal to those who have challenged him. On Fox News Sunday, the real estate mogul called Cruz "a little bit of a maniac" who would not be able to get anything done as president, given fellow senators' disdain for him.

Still, with a commanding lead in Iowa polls, a strong performance Tuesday by Cruz could set him up well to win the caucuses and puncture Trump's winning aura.

One thing is clear: the nonaggression pact the two have had is in danger. As party leaders' concerns and talk of a possible deadlocked convention have grown, Trump has threatened to run as an independent.

The two-hour debate, at the Venetian hotel and casino, will be televised live by CNN, with programming beginning at 8:30 p.m., moderated by anchor Wolf Blitzer. (An earlier, one-hour debate among lower-polling candidates will be broadcast at 6 p.m.)

These are the first Republican debates since the terrorist attacks in Paris and San Bernardino, Calif., and national security questions are expected to dominate. Among the issues: Trump's proposal to temporarily ban Muslims from entering the country, as well as strategies for combating the Islamic State terrorist group in Syria and Iraq, including whether U.S. ground troops ought to be deployed and, if so, in what numbers.

Gov. Christie will be on the main debate stage, after a turn last month in the "undercard" debate, and will seek to capitalize on recent momentum, particularly in New Hampshire, which holds the first primary a week after Iowa inaugurates the nomination contest with its caucuses.

Kentucky Sen. Rand Paul, who began the campaign as an intriguing figure with his libertarian-flavored appeal, will be fighting for his political life. He barely made the main debate under CNN's poll-based criteria.

Other candidates in the main debate: retired neurosurgeon Ben Carson, Florida Sen. Marco Rubio; former Florida Gov. Jeb Bush; former Hewlett-Packard CEO Carly Fiorina; and Ohio Gov. John Kasich.

Participating in the earlier debate will be former Pennsylvania Sen. Rick Santorum; former Arkansas Gov. Mike Huckabee; South Carolina Sen. Lindsey Graham; and former New York Gov. George Pataki.

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GOP Debate

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When: 8:30 p.m Tuesday. Undercard will be at 6.

Where: Las Vegas

TV: CNN

Moderators: Wolf Blitzer, Dana Bash, Hugh Hewitt.

Main debate: Donald Trump, retired neurosurgeon Ben Carson, Texas Sen. Ted Cruz, Florida Sen. Marco Rubio, former Florida Gov. Jeb Bush, former Hewlett Packard CEO Carly Fiorina, New Jersey's Gov. Christie, Ohio Gov. John Kasich, and Kentucky Sen. Rand Paul.

Undercard: former Arkansas Gov. Mike Huckabee, former Pennsylvania Sen. Rick Santorum, South Carolina Sen. Lindsey Graham, and former New York Gov. George Pataki.

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