The Republican presidential candidates arrive in Las Vegas for a debate Tuesday night in a race in which new feuds are poised to erupt, and the focus of which has shifted dramatically toward national security. 

There will be plenty for the candidates to chew on after an eventful month since the top contenders last shared a debate stage, with no issue getting more attention than terrorism and foreign policy after attacks killed or injured about 500 in Paris and another 36 in San Bernardino, California.

In the wake of the two attacks, Florida Senator Marco Rubio has pushed his plan for more military spending and highlighted his differences with some in the field for more aggressive intelligence programs. During a Monday rally in Las Vegas, Rubio suggested that the U.S. government should be inspecting social media accounts before allowing immigrants into the country, saying that's where many pledge allegiance to terrorist groups like the Islamic State.

"The threat we face is significant, it's different than any other threat we've ever faced," Rubio told a crowd of about 200 in a Renaissance Las Vegas meeting room.

But Rubio's push for robust surveillance programs, which has pitted him against Texas Senator Ted Cruz, has taken a back seat to front-runner Donald Trump's brazen call to block entry into the United States for all Muslims. Trump's plan, which is sure to be a topic of debate on Tuesday, has been denounced by Republicans and Democrats alike, but was supported by nearly two-thirds of likely 2016 Republican primary voters, according to a Bloomberg Politics poll last week. More than a third said they were more likely to vote for him.

The spotlight during the fifth Republican debate will also be focused on Cruz, who has leaped to the lead in Iowa. Cruz's climb to the top of the field there comes as he has battled Rubio over immigration and national security. Trump has also taken notice of Cruz's rise, but brushed off his rival's numbers at a pre-debate rally Monday night at the Westgate Las Vegas Resort & Casino.

"You know the bottom line? I think we're going to win Iowa," Trump told a crowd of a few thousand cheering supporters. "I think we're going to win New Hampshire big. I think frankly, if we win Iowa, we're going to run the table."

The bond between the celebrity real estate mogul and the first-term senator from Texas looks to be tenuous, however, after Trump called Cruz a "maniac" in an interview with Fox News Sunday, in which he described the senator as having no Washington accomplishments. 

"Trump is feeling the heat in Iowa as Cruz gains on him," said Ryan Williams, a Republican strategist. "But the test for Cruz is whether he can attack Trump to pick up some of his supporters without making them angry."

Williams, a former spokesman to 2012 presidential nominee Mitt Romney, said that "most people are going to watch to see if Cruz and Trump mix it up."

"Clearly, the Trump campaign wants to target Cruz, and Cruz wants to deflect," Williams said. 

"Existential threat"

A flare up between Cruz and Trump, the top two candidates in national polls, would delight the traditional pro-business wing of the party, which has little patience for either candidate. The Republican establishment lined up early behind former Florida Governor Jeb Bush, but influential donors including New York hedge fund manager Paul Singer have been gravitating to Rubio as the son and brother of former presidents has faltered.

Choosing between Trump and Cruz "would be tough to choose because they are going to have a very difficult time or will refuse to put together a coalition beyond blue collar white Republicans that are angry about where the country is headed," said Ron Bonjean, a Republican strategist and former top congressional aide.

Rory Cooper, a Republican strategist in the "anybody but Trump" camp, indicated that Cruz would be preferable to the blustery Trump.

"I would gladly vote for whoever is polling last in the latest poll before voting for Donald Trump," Cooper, a former communications director for Eric Cantor wrote in an e-mail. "This is about the long term health of the Republican party and conservatism, and Trump is the only existential threat to either."

Ed Rogers, a Republican lobbyist and veteran strategist, also prefers Cruz to Trump.

"He is a real Republican," Rogers wrote in an e-mail. "Proven commitment to conservative principles. Not irrational or an egomaniac."

So far, debate performances have had limited impact on the candidates. Rubio enjoyed wide praise for four smooth debates, but none have earned him a clear surge of support. Trump has consistently been panned by prominent conservatives for what some consider bizarre outings bereft of policy specifics, yet he has continued to dominate most polls.

Former Florida Governor Jeb Bush's reasonably strong fourth debate did little to help his precarious standing in the polls. Carly Fiorina's attack on Planned Parenthood in the second debate garnered her enough support that she rose to the top three, but she has since fallen in the crowded field.

Gloves off

With less than two months remaining before the Feb. 1 Iowa caucuses and the Feb. 9 New Hampshire primary, middle-of-the-pack candidates hoping for a late surge in the polls have little choice but to come out swinging on Tuesday. That combative posture has not, however, come easily for Bush, retired neurosurgeon Ben Carson, or Ohio Governor John Kasich, who said in an interview with Face the Nation on Sunday that "polarization and divisions is going to lead us down the wrong path." According to the Real Clear Politics polling average Kasich, Bush, and Carson stand in fifth, sixth place, and seventh place, respectively, in New Hampshire, a state where Kasich and Bush have invested significant resources and time. 

Polling in third place in New Hampshire, New Jersey Governor Chris Christie over the weekend took aim at Rubio, one of the two men he still trails there, over his poor attendance record in the senate, as well as on the campaign train in the Granite State. “He’s never here,” Christie said at a New Hampshire town hall on Friday. 

Rubio, meanwhile, previewed another line of attack on Monday against Cruz and other rivals by claiming to be the only Republican in the race who has accomplished anything concrete in the fight against Obamacare. 

“Everybody running for president as a Republican is against Obamacare,” Rubio said at his Las Vegas rally. “But I’m the only one running that’s ever scored a victory against Obamacare.”

Rubio last year pushed for a provision that eliminated the so-called “risk corridors” in the Affordable Care Act, which help cover the costs of those with higher health risks. The change, attached to a government spending measure, prevents the government from paying out insurers, but could also lead to higher premiums and less choice for consumers.

Within hours of the fourth debate on Nov. 10, Cruz picked a fight with Rubio over the Floridian's work with Democrats on an immigration bill in 2013 that included a path to citizenship for undocumented workers, which Cruz routinely lambastes as "amnesty." The fight could flare up Tuesday as the issue is a vulnerability for Rubio as surveys say a majority of Republicans would rather deport people in the U.S. illegally than give them legal status.

Raul Espinosa, a 62-year-old retiree in Henderson, Nevada, said he hopes the debate will help him choose between Rubio and Cruz, adding that he admires Rubio’s passionate speeches about the American Dream and devotion to free-market capitalism. “The problem is I’m listening to Ted Cruz,” Espinosa said after watching Rubio speak Monday in Las Vegas. “I was shocked Marco Rubio was part of the Gang of Eight. He should know better on immigration. I was disappointed in him.”

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