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For the candidates, the 90-minute debate will be their first opportunity to present their cases to a large audience. In a dauntingly brief time, they'll each try to explain who they are, display some gravitas and find a way to stand out from the crowd.
Here's a quick viewer's guide to the candidates:
Sen. Marco Rubio (R-Fla.) has offered more policy specifics — so many that he sometimes comes across as wonky — but some conservatives are wary of his past support for bipartisan immigration reform. He'll try to steer the conversation toward foreign policy.
Sen. Ted Cruz (R-Texas) may be the most polished debater in the bunch; he was a college champion at Princeton. He's been careful to avoid criticizing Trump's intemperate statements, in an apparent attempt to inherit the mogul's supporters if he drops out of the race. If Trump is attacked Thursday, will Cruz rise to his defense?
Three candidates are casting themselves as different kinds of Republicans.
Sen. Rand Paul (R-Ky.), still working to expand his father's libertarian camp in the GOP, has been forthright in criticizing his rivals as too willing to take the United States to war.
Ohio Gov. John Kasich, the most moderate Republican in the race, has called for more attention to the poor and the downtrodden — "those who live in the shadows," in his phrase.
And Ben Carson, a retired neurosurgeon, has managed to be both incendiary (comparing Obamacare to slavery) and sensible (arguing that Republicans should replace Obamacare before they repeal it); which Ben Carson will show up?
Two more candidates have been struggling to recapture the excitement their names commanded in earlier campaigns.
Mike Huckabee has tried to appeal to Christian conservatives, but many haven't responded. New Jersey Gov. Chris Christie has reached out to fiscal conservatives, calling boldly for cutting Social Security and Medicare benefits for the wealthy, but the only word many voters seem to associate with Christie is "Bridgegate."
And then there's Trump. There's actually a way he could win this debate — by treating it seriously. That's what he says he'll do. "It is certainly my intention to be very nice and highly respectful of the other candidates," he tweeted last week.
If another candidate attacks him first and he responds evenly, he wins.
But some kind of confrontation is virtually guaranteed for one simple reason: This isn't a civics exercise, it's commercial television.
"Trump, you know he wants to go after people," Fox anchor Chris Wallace told the Washington Post. "And you can be sure there will be a moment … where somebody is going to give him a fat juicy ball right in there so he can go after Bush, and see how he responds to it."
What's less certain is how much real substance will emerge. A 90-minute debate with 10 participants means each candidate will have less than nine minutes to speak.
Still, viewers may well discover that, as a group, the GOP candidates are more talented than they knew. It's a much stronger field than the one Mitt Romney faced in 2012 (his top rivals were