WASHINGTON — The vote that may have the greatest impact on determining the Democratic presidential nominee may not be the elections held yesterday in Indiana and North Carolina or the one next week in West Virginia, but rather one taken by 30 party insiders set to meet here at the end of the month.

Those Democratic leaders, who form the national party’s Rules and Bylaws Committee, will decide on official challenges to the penalties levied against Florida and Michigan, whose delegates to the August nominating convention were stripped last year after they scheduled early primaries in violation of party rules.

Their verdict looms increasingly large as the battle for the nomination drags on. Senator Clinton must now decide whether to fight on after holding what appeared last night to be a narrow lead in the Indiana primary and losing decisively to Senator Obama in North Carolina.

With 84% of the precincts reporting, the New York senator led in Indiana, 52% to 48%. Mr. Obama held a much wider advantage in North Carolina, 56% to 42%, with 79% of the precincts reporting.

Yesterday’s elections are likely to widen his lead in delegates, making Mrs. Clinton’s road to the nomination significantly steeper. A former Tennessee congressman and the head of the Democratic Leadership Council, Harold Ford Jr., said on MSNBC that Mrs. Clinton must “think long and hard” about whether to contest the next primaries.

Addressing supporters in Raleigh last night, Mr. Obama cast his North Carolina win as a validation of his anti-Washington message. “Some were saying that North Carolina would be a game-changer in this election,” he said. “But today, what North Carolina decided is that the only game that needs changing is the one in Washington, D.C.”

The rules committee will convene in Washington on May 31 in a public hearing at a yet-to-be-determined location. Officials expect the meeting to last for a full day or more.

Mrs. Clinton’s campaign wants the panel to reinstate the Florida and Michigan delegations according to the elections the two states held in January, which she won. Neither candidate actively campaigned in Florida, and Mr. Obama removed his name from the ballot in Michigan.

Such a move would significantly cut into Senator Obama’s delegate lead and boost Mrs. Clinton’s currently slim chances of wresting away the nomination with the help of superdelegates.

Yet it would also raise the specter of a backroom deal that could raise doubts about the legitimacy of Mrs. Clinton’s victory and spur a revolt by Obama supporters, particularly African Americans.

Florida’s representative on the Rules Committee, Allan Katz, an Obama backer, warned the panel against taking any action that could affect the outcome of the nomination.

“I think that would be institutionally a tragic thing to do, and I think that it would be tantamount to stealing the nomination,” Mr. Katz, a Tallahassee city commissioner, said yesterday in a telephone interview.

Adding to those concerns is the significant number of Clinton supporters who sit on the committee. Of the 30 members, 13 are backing the former first lady, eight are supporting Mr. Obama, and the remaining nine have not publicly endorsed either.

One of the members is a longtime Clinton adviser and her chief delegate strategist, Harold Ickes, and another is the chairman of the Michigan Democratic Party, Mark Brewer, who is neutral but has pushed for the seating of his state’s delegates at the convention.

The panel’s two co-chairmen are a former labor secretary under President Clinton, Alexis Herman, and James Roosevelt Jr., the grandson of President Franklin Delano Roosevelt. Neither has endorsed a candidate.

Heading into last night’s primaries, Mr. Obama led Mrs. Clinton by 137 total delegates and stood 280 short of the 2,025 needed to clinch the Democratic nomination, according to the Associated Press tally. Florida and Michigan carry more than 350 delegates between them, and awarding them based on the January election results could give Mrs. Clinton a net gain of more than 50, according to some estimates. The Clinton campaign argues that if Florida and Michigan are included, the new magic number of delegates to secure the nomination would be 2,209.

Both the Clinton and Obama campaigns have said the delegations from each state will eventually be seated, as has the chairman of the Democratic National Committee, Howard Dean. Florida and Michigan are key to the party’s hopes of winning in November, and the Republicans are already eager to use the delegate fight to win over voters.

Dr. Dean said yesterday that he would support a resolution that acknowledged the more than 2 million voters who went to the polls in January, that was fair to both candidates, and that respected the “48 states that followed the rules” in scheduling their primaries.

“There will be some sort of compromise in the rules committee on the 31st of May, and I hope [that] will result in seating delegations from Florida and Michigan,” Dr. Dean said in an appearance on CNN. “But we don’t know what that compromise is going to look like right now.”

Supporters of each candidate as well as officials in Florida and Michigan have floated a variety of proposals for resolving the dispute.

The official challenges filed with the Rules Committee ask that each state be awarded half of the delegates it would have gotten if it followed the rules, arguing that the DNC’s punishment for holding early elections exceeded the maximum penalty under party rules, which would be 50% of a state’s delegates. The states also want their superdelegates reinstated, alleging that nothing in the party bylaws allows for them to be stripped.

“They have rules about you take a certain percentage of delegates away. There’s nothing in the rules that say you take a machine gun or shotgun and take them all,” the Michigan DNC member and Clinton supporter who filed that state’s appeal, Joel Ferguson, said in a telephone interview yesterday.

The Obama campaign has suggested the delegates be split 50-50 between the two campaigns, but Mr. Ferguson –and the Clinton campaign — say that is unacceptable because it would ignore the will of Democrats who turned out in January.

“Michigan did not go first to be irrelevant for the convention,” Mr. Ferguson said. “So deciding to seat the delegates after the decision has been made is no victory for anyone.”

A group led by Senator Levin of Michigan has put forward a solution that would net Mrs. Clinton some delegates but not as many as if the election results were fully recognized. The committee is free to consider any idea, which, to be adopted, would require a majority vote.