LAS VEGAS (AP) -- Herman Cain has risen fast. Now the question is: Will he fall?

The businessman has been facing intense scrutiny in recent weeks as his poll numbers have soared. He likely will face tough questions - on his professional past and his issue positions as well as his viability - when he gathers on stage later Tuesday with his Republican presidential foes for the latest GOP debate.

Cain is the latest GOP hopeful to enthrall the party faithful who haven't rallied behind Mitt Romney.

The former Massachusetts governor is seen as the Republican to beat. He has turned in strong debate performance after strong debate performance. He's fundraising steadily and adding establishment supporters nationally and also in early primary states.

But that hasn't kept the party's base from searching for an alternative.

Minnesota Rep. Michele Bachmann had a quick rise - after a sharp debate performance in New Hampshire - and a quick fall after she won a key test vote in Iowa. And Texas Gov. Rick Perry was embraced by most Republicans when he announced his bid in August, but has faded after a series of missteps.

Cain, a former pizza company executive, says he's different than other candidates who've faded, and that his years of speaking to tea party rallies and other conservative groups have built a grassroots support base.

His years spent public speaking, hosting a radio talk show and aligning himself with groups like Americans for Prosperity - whose funding comes in part from the billionaire Koch brothers - boosted his profile in conservative circles. But those appearances also created a huge record of statements on issues that rivals are certain to mine if they start to believe that he has staying power - and look for ways to derail him.

Since becoming a national figure, Cain's already run into the kind of trouble he was able to avoid before when he wasn't in the spotlight.

His tax plan - which he called 9-9-9 - already has been derided by Democrats and Republicans alike. The plan would scrap the current tax code and replace it with a 9 percent tax on personal income and corporations as well as a new 9 percent national sales tax.

Over the weekend, Cain acknowledged that he was "not familiar" with neoconservatism during an appearance on NBC's "Meet the Press." Cain identified two foreign policy hands he admires - John Bolton and Henry Kissinger - but those two men have fundamentally different philosophical approaches.

And Cain also had to apologize for comments he made over the weekend calling for an electric fence on the Southern border with Mexico.

At a campaign stop Monday in Arizona, Cain appeared with Maricopa County Sheriff Joe Arpaio, an aggressive anti-immigration proponent.

"It was a joke," Cain said emphatically during a news conference. "I apologize if I offended anyone. Mea culpa, mea culpa, mea culpa."

Cain had told an audience in Tennessee that the fence is "going to be electrified. And there is going to be a sign on the other side that says, `It will kill you.'"

Immigration already has flared on the campaign trail - and has contributed to the sinking of another fast-rising GOP candidate.

Perry has struggled to explain why he signed a law giving in-state tuition breaks to illegal immigrants at Texas universities.

Also participating in Tuesday's debate are Romney, Perry, Bachmann, Texas Rep. Ron Paul, former Pennsylvania Sen. Rick Santorum and former House Speaker Newt Gingrich.

Former Utah Gov. Jon Huntsman wasn't planning to be on stage; he's boycotting the Nevada caucuses in defense of New Hampshire's first-in-the-nation primary. Nevada has scheduled its contest for Jan. 14, and Republican officials are pressuring Romney and other Republicans to join Huntsman's boycott if the state refuses to hold the caucuses later in January.

Romney has faced pressure from New Hampshire leaders to join that boycott.

Also potentially at issue on Tuesday is the foreclosure crisis.

So far, it's been almost forgotten on the campaign trail, but the candidates will probably have little choice but to address it. Nevada has the nation's highest unemployment rate, a statistic that's driving the highest foreclosure rate in the nation. It's the root of the economic crisis, but it barely has been discussed as issues like immigration and vaccines for children have dominated the GOP primary.

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