WASHINGTON — The Republican presidential candidates will face off in a debate for the second time tomorrow in South Carolina, but forgive the leading contenders if they aren’t exactly excited about the showdown.

If the first debate earlier this month in Southern California showed anything, it was that the top-tier candidates have as much to lose as they stand to gain. While a former Massachusetts governor, Mitt Romney, was widely praised for his performance, Mayor Giuliani is, 10 days later, still facing questions about his responses on abortion.

With substantive but occasionally stilted and rehearsed answers, Senator McCain did well enough to hold his position among the leaders, but he did little to advance.

The format is largely unchanged for tomorrow’s debate, which will take place on the University of South Carolina campus in Columbia. Ten candidates will answer questions for 90 minutes on just about any topic; the event will be broadcast live on Fox News, with anchor Brit Hume serving as moderator.

While the format affords longshots — such as a Texas congressman, Rep. Ron Paul, and a Colorado lawmaker running on an anti-immigration platform, Rep. Tom Tancredo — a rare opportunity to reach a national audience, the leading campaigns are grumbling. Candidates have 30 seconds or a minute to answer questions, giving them little time to provide details or fully explain their views.

Aides to the candidates say the format is out of their hands, decided by the state parties and the cable networks that sponsor the events.

“The debate rules are decided by the hosts, and the campaign simply looks for an even playing field,” an adviser to Mr. McCain said.

Despite the complaints, the campaigns say they have few options. Skipping out is not one of them, as even the leading candidates are looking for every opportunity to reach voters at this early stage in the race.

“You can’t wring your hands about what would be the best debate,” a Romney spokesman, Kevin Madden said. “We are still in the creating awareness stage for Governor Romney.”

The eligibility requirements for participating in tomorrow’s debate were minimal. Candidates had to have formed an exploratory committee, paid the fees to appear on the South Carolina primary ballot, and garner at least 1% in state and national polling, as conducted in early April by Fox News and Opinion Dynamics.

The chairman of the state Republican Party, Katon Dawson, said the cable network was chiefly responsible for deciding the format, but he defended the decision to invite 10 candidates.

“Right now, early in the debate process, I think it was very important to be inclusive and not exclusive, and that’s what we’ve done,” he said in an interview. The party will host at least one other debate in the state shortly before its primary — the first GOP contest in the South — which had been scheduled for early February but could be moved up. Mr. Dawson said he expected the field to be narrowed by then.

The level of inclusiveness hasn’t satisfied every potential White House aspirant. A Chicago businessman, John Cox, filed a lawsuit last week to stop the South Carolina debate, saying organizers had rigged the criteria to exclude him. A federal judge on Friday dismissed his claim, the Associated Press reported.

After tomorrow, the next set of presidential debates will be June 3 for Democrats and June 5 for Republicans in New Hampshire, each sponsored by CNN and local news organizations. The leading Democrats committed to the debate earlier this month, albeit reluctantly.

Senator Obama of Illinois, whose performance in the first Democratic debate was criticized as shaky by some analysts, was the last to accept the invitation.

“One of the challenges of the debates is that they are often formatted to support sound bites and not explanations of solutions,” a campaign spokeswoman, Jennifer Psaki, said. “But Senator Obama embraces the opportunity to discuss issues of importance like Iraq, healthcare, and ethics and show the American public that he represents the kind of change this country needs.”

While the Democrats have agreed to monthly party-sanctioned debates beginning in July, the leading Republicans have not yet committed to debates beyond the June 5 date. Without a change in the 10-candidate format, Messrs. Giuliani, McCain, and Romney may be hesitant to do so.

For any contender, the direct impact of a debate is less in the number of potential voters who actually tune in — relatively few — than in the consensus that emerges among pundits and the press. In the first debate, the winner appeared to be Mr. Romney, who has raised more money than his rivals but is less well-known and only recently has begun to rise in the polls. “He got very positive spin coming out of the debate,” a political science professor at Boston University, Tobe Berkovitz, said.

In tomorrow’s GOP debate, the greater scrutiny will likely be on Mr. Giuliani once again, particularly on the issue of abortion. After saying in the first debate that it would be “okay” if the landmark Roe v. Wade decision were overturned, he has spent the last week explaining his support for abortion rights in interviews and in a speech in Houston.