The Republican presidential hopefuls debate in Las Vegas Tuesday night will be the first since the terrorist attacks in Paris and in San Bernardino, Calif. In recent weeks, ISIS and how to keep Americans safe have dominated the campaign and shot to the top of Americans' concerns.

An NBC/Wall Street Journal poll this week found that 40 percent rated national security and terrorism as what should be the government's top priority, double those who said so in April. The issue has been a top one for Republicans for some time. But Quinnipiac found that while in November, Iowa Republicans rated the economy and jobs as the top issue with terrorism second, since the Paris and San Bernardino attacks, that has flipped.

The last debate — back on Nov. 10 — focused mostly on the economy. But days later, 130 were killed in terrorist attacks in Paris. Then, just weeks later, on Dec. 2, another jolt.

A radicalized husband-and-wife couple killed more than a dozen in San Bernardino at an office party. Both shooters were Muslim. One was an American citizen; his wife came to the U.S. on a fiancee visa.

Immediately, GOP candidates pointed fingers at President Obama, saying he didn't take terrorist threats and ISIS seriously enough.

After San Bernardino, Donald Trump called for stopping all Muslims from entering the U.S., at least temporarily — "until our country's representatives can figure out what the hell is going on."

Such tough talk from Trump was nothing new. There was his expletive-laden call to bomb ISIS even before Paris. Amid all of this Trump surged in national polls of GOP voters.

Other candidates, including Marco Rubio, Jeb Bush, Chris Christie and John Kasich, called Trump unhinged, offensive and ridiculous.

Ted Cruz — who hopes to win over Trump supporters when, as he expects, Trump fades — was more mild in his critique.

"No, that that that is not my policy," he told NBC. "I believe the focus should be on Islamic radical terrorism."

So that's the backdrop for tonight's debate.

Trump remains on top. Ben Carson has slipped after several stumbles on foreign policy. Gaining ground is Cruz, who suddenly has a big lead in the new Des Moines Register poll in Iowa, considered the gold standard in the state. His rise is being fueled there by religious conservatives, a group he has carefully courted.

So Trump is now going after Cruz.

"I don't think he's qualified to be president," Trump said on Fox News Sunday, explaining, "because I don't think he has the right temperament. I don't think he's got the right judgment."

Trump got an opening from audio leaked to the New York Times of a Cruz fundraiser in which Cruz questions Trump's seriousness and judgment.

"Who am I comfortable with their finger on the button?" Cruz is heard saying in one part.

There's also the chance for a triangle of fire. The always-on-message Marco Rubio, who serves on the Senate Foreign Relations Committee, could go after Cruz for his support of getting rid of the NSA's metadata program.

And that sets the stage for a top-tier showdown.

Tuesday night is also, however, important to another candidate who has gotten a boost since the last debate: Chris Christie. The New Jersey governor was endorsed by New Hampshire's biggest newspaper, the Union Leader. Trump still leads in that state, but Christie sees an opportunity.

On Michael Medved's radio show, he dismissed Trump's proposed ban on Muslims.

"There are folks in this race who don't care about what the law says, because they're used to being able to just fire people indiscriminately on television," Christie jabbed.

In all, there will be nine candidates on the main stage; among them, Ohio Gov. John Kasich, who has been Trump's most persistent critic in the race. A superPAC supporting Kasich has a new Web ad that tries to deflate Trump's blue-collar appeal. It features Trump — and a flatulent hippopotamus.

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