WASHINGTON — An experiment in presidential debating that allows Web users to ask the questions could hasten calls for a format that winnows down the field of candidates for a more substantive policy exchange.

The Democratic contenders are gathering tonight in Charleston, S.C., for the first “YouTube debate,” in which they will field queries that originate not from a crisp-suited network moderator or panel of journalists but from Americans across the country who have filmed themselves in their living rooms, offices, or even bedrooms. While debate organizers are billing the event as a novel approach aimed at engaging a new generation of voters, the format has drawn criticism from some who say it could cheapen a campaign discourse already dominated by sound bites.

The two-hour event, which CNN is broadcasting live beginning at 7 p.m, also includes the same eight candidates who have taken part in three previous debates and other forums — despite grumblings from some political observers, and more recently, from Senator Clinton and John Edwards, that the number of participants should be reduced.

Mrs. Clinton and Mr. Edwards were caught on microphone after an NAACP candidate forum earlier this month suggesting to each other that their campaigns discuss ways of limiting the number of candidates at future events. The flap drew immediate protest from other candidates, such as Rep. Dennis Kucinich of Ohio, for whom the monthly debates are virtually the only chance to reach a national audience. Mrs. Clinton later said it was Mr. Edwards’s idea, while the former North Carolina senator said he did not want to eliminate contenders but instead recommended separating the debate field into two groups of four, randomly selected, to have more in-depth policy discussions.

As the front-runner, Mrs. Clinton likely would have little strategic interest in smaller events; she has won rave reviews for her performances in the initial debates, and more intimate gatherings would allow her closest competitors, Mr. Edwards and Senator Obama of Illinois, to engage her directly.

Any effort to cut candidates out of debates will be difficult. A spokesman for the Democratic National Committee said yesterday that all of its sanctioned debates, of which tonight is the first, will include the eight contenders. The leading Democrats have all embraced the YouTube format, at least outwardly, by encouraging their supporters to submit questions. But in a sign of protest, Mr. Edwards has scheduled a separate 30-minute Web cast following the debate to answer voter questions and give what his campaign says will be “in-depth, substantive answers — not 60-second sound bites — on the critical issues facing our country.”

The Republican campaigns will also be watching closely; they have agreed to a YouTube debate in September.

While the candidates will not know in advance the questions that will be picked, they can see the vast range of more than 2,500 videos that users have posted on the YouTube site.

And quite a range it is.

Many viewers have posted serious questions, asking for the candidates’ positions on Iraq, what they think the national minimum wage should be, or how they think federal education dollars should be spent.

Some queries are more provocative: In a video sent from Murcia, Spain, Steve Marcs demands that the candidates name the one other contender on the stage that they would support for president. Other video queries border on the embarrassing and even crude. A shirtless man, John Dardenne of Baton Rouge, La., cites the “precedents set by previous Clinton administrations” and asks of Mrs. Clinton, “Could I be your intern?”

Another video focuses on a Batman action figure, with a male voice first asking candidates how they would lower urban crime before digressing into a description of his infatuation with the former first lady.

It is submissions like these that has critics, if not the candidates themselves, worried that the debate could make a mockery of the political discourse.

“I think it’s pathetic and continues to debase the media’s role in campaigns,” the acting dean of Boston University’s College of Communication, Tobe Berkowitz, said. “It’s basically a game-show format.”

CNN officials will choose the questions, and while it is safe to say they will probably not pick a video that would embarrass or offend a candidate, neither are they committed only to serious questions. Anderson Cooper will moderate the debate and have leeway to ask follow-up questions, a network spokeswoman said. About 25–30 videos are expected to be shown over the course of two hours.

“I think we’re going to see some of the standard questions. We may hear them with a more personal angle,” a political scientist at Hunter College, Andrew Polsky, said. He predicted CNN would throw in a few “odd questions” but would stay away from any that are too esoteric or overly probing.

“There’s an interesting tension between attempting to create an unscripted moment and the determination of candidates to remain on message,” he said, adding that the candidates usually have the edge. “I’m not sure if this format will be effective in breaking this down.”