DAVID GREENE, HOST:
The Republican presidential field just got smaller. Well, at least the field you will see on stage if you tune into tonight's primetime debate. There were strict qualifying rules to be on the main stage in Charleston, S.C. And some candidates have been relegated to a so-called undercard debate. One of them, Senator Rand Paul, says he's just not going to show up. Here's NPR's Don Gonyea.
DON GONYEA, BYLINE: To make the main event, you needed to in the top six in national polls or top five in either Iowa or New Hampshire. The Fox Business Network is hosting. So here's the lineup. Right in the middle of the stage, Donald Trump, flanked by Ted Cruz and Marco Rubio, then Ben Carson and Chris Christie, with Jeb Bush and John Kasich on the ends. Bumped from the main stage this time are Carly Fiorina and Rand Paul. Paul is saying no, thanks to the undercard debate to be held earlier tonight. He spoke to CNN.
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RAND PAUL: It really sort of points fingers and says, well, are you really going to be a contender? We are a contender. We think we have a national campaign that can contend for victory. And we can't accept sort of an artificial designation by anybody.
GONYEA: Fewer participants means more precious minutes for each candidate to answer questions or to challenge statements by others on stage. They seem primed to do just that. Let's start with Trump. He's the national front-runner, but Cruz leads in Iowa. So Trump has been jabbing at Cruz lately.
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DONALD TRUMP: So I'm running against different people. And Ted's been nice to me. And I've been nice to him. But he's got a problem, and you know he's got a problem.
GONYEA: This is from a rally in Iowa this week. Trump is talking about Cruz's birth 45 years ago in Canada. Cruz's mother was American, but Trump still says Cruz may not be eligible to be president.
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TRUMP: I don't know. What do I know? But I can tell you this. The Democrats are going to bring the suit. And you can't run unless you're going to be - how can you run like that? You don't even know.
GONYEA: So what Trump tried to do to President Obama by challenging his birth certificate he's now doing to Cruz. Except Cruz really was born outside the U.S. Cruz has dismissed the idea that he's not eligible. On the syndicated "Howie Carr Show," Cruz was asked about Trump suddenly playing "Born In The U.S.A." at his rallies.
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TED CRUZ: Well, look. I think he may shift in his new rallies to playing "New York, New York," because, you know, Donald comes from New York. And he embodies New York values. And listen, the Donald seems to be a little bit rattled.
GONYEA: Because Trump leads in New Hampshire, just about everyone on stage may be feel compelled to confront him tonight. Trump might not be the only focus, though. Senator Marco Rubio could again have to explain why he worked with Democrats on an overhaul to the nation's immigration laws. A super PAC backing Cruz has been attacking Rubio with this web ad.
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UNIDENTIFIED MAN #1: He was a co-author of the bill. I mean, it was the Rubio bill. It was the Rubio-Schumer bill.
UNIDENTIFIED MAN #2: His fingerprints are all over that bill.
GONYEA: Rubio says such attacks are a result of his connection with voters. Here he is on ABC News.
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MARCO RUBIO: Well, we're going to keep doing what we're doing now. Look, there's a lot of voters in these early states, particularly in Iowa but also in New Hampshire, that are going to make their decisions tvery late. They're still shopping. You can see it. You can sense it in your conversations.
GONYEA: And many of them will be watching tonight's debate, as voting fast approaches in those places. Don Gonyea, NPR News, Washington.