The inaction in Michigan followed a similar collapse of her effort to seek another matchup with Mr. Obama in Florida, where, as in Michigan, she won an earlier primary held in violation of party rules.

Without new votes in Florida and Michigan, it will be that much more difficult for Mrs. Clinton to achieve a majority in the total popular vote in the primary season, narrow Mr. Obama’s lead among pledged delegates or build a new wave of momentum.

Mrs. Clinton’s advisers had hoped that the uproar over inflammatory remarks made by Mr. Obama’s longtime pastor that has rocked his campaign for a week might lead voters and superdelegates to question whether they really know enough about Mr. Obama to back him. Although it is still early to judge his success, the speech Mr. Obama delivered on race in Philadelphia to address the controversy was well received and praised even by some Clinton supporters.

Tad Devine, a Democratic consultant who is not supporting a candidate, said Mrs. Clinton faced a challenge that although hardly insurmountable was growing tougher almost by the day. Mr. Devine said it was critical for her to come out ahead in popular votes, cut into Mr. Obama’s lead and raise questions about Mr. Obama’s electability to win over superdelegates.

“They are going to have to be flawless in executing the strategy, which achieves the goal of taking away the advantage Obama has in pledged delegates and the popular vote,” he added. “Any major setback could undercut that goal. Obama is in the advantageous position.”

The race is certainly not over. With 10 contests remaining, Mrs. Clinton trails Mr. Obama by about 150 delegates out of the 2,025 needed to secure the nomination.

If there is a road to victory for Mrs. Clinton, it is a fairly narrow one. Her chief strategist, Mark Penn, said the campaign believed that when the primary voting was done, Mrs. Clinton would have a lead in the overall popular vote, that Mr. Obama’s lead in delegates would be relatively narrow and that polls would show her in a stronger position than Mr. Obama.

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Victories in contests where she is strong or competitive like Pennsylvania, West Virginia, Puerto Rico and, perhaps, Oregon and Indiana could give her a burst of energy.

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No less important, the campaign hopes that Mr. Obama will have been battered by five rough weeks that raise questions about his past, including the pastor’s incendiary comments, that would underscore Mrs. Clinton’s warning to Democrats that they were rallying around someone who was untested and unvetted.

“The superdelegates are not going to really decide until June,” Mr. Penn said. “He’s just going through a vetting and testing process that didn’t happen a year ago and is now happening. The whole vetting and testing process will make a big difference.”

It is in the interest of Mrs. Clinton’s campaign to portray the contest as being highly competitive. Her campaign is intent on combating Mr. Obama’s efforts to pick off superdelegates. And it is increasingly concerned that any sign that the window is closing could lead a Democrat like Al Gore or Speaker Nancy Pelosi to step in and urge Democrats to back Mr. Obama in the interest of unity.

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In truth, in interviews, Mrs. Clinton’s advisers said that task was tough and growing tougher and that the critical questions were what would happen with Florida and Michigan and the possibility of developments involving Mr. Obama’s relationship with his spiritual adviser, the Rev. Jeremiah A. Wright Jr.

The fight over Florida and Michigan is just partly about delegates. Victories in new primaries in those states are among the only realistic ways for Mrs. Clinton to erase Mr. Obama’s advantage in the total popular vote.

Mr. Obama’s edge over Mrs. Clinton is 700,000 votes out of 26 million cast, excluding caucuses and the disputed Florida and Michigan results. About 12 million people are eligible to vote in the remaining contests.

Aides to the two candidates said even with the best possible showing for Mrs. Clinton in the states ahead, it was hard to see how she could pass Mr. Obama without Michigan and Florida.

She received 300,000 more votes than Mr. Obama in Florida in January. In Michigan, where none of her major opponents were on the ballot, she drew 62,220 more votes than the rest of the opponents. Mrs. Clinton’s advisers said that absent some deal to seat the delegates from those states, the campaign would still argue that the popular vote in Michigan and Florida be counted.

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“The popular vote is the popular vote for all to see,” said Harold Ickes, a senior adviser to Mrs. Clinton. “For people to claim that because the delegates weren’t seated you can’t count the popular vote seems somewhat goofy.”

Yet that could be a tough argument to make. None of the candidates campaigned in Michigan or Florida, and Mr. Obama’s name did not appear on the Michigan ballot.

Finally, Mrs. Clinton’s aides hope that disclosures about Mr. Obama’s past like the one involving Mr. Wright could give superdelegates’ pause. Mr. Devine said he thought that at least in terms of Democratic primary voters Mr. Obama had turned the furor to his advantage with his speech on race.

“Obama, confronted by an issue that was boiling, seemed to wade into it with a speech that was in many ways profound,” Mr. Devine said. “As a result, now these people who were so interested and awakened by his candidacy are back with him again. Instead of this being a setback, it becomes an opportunity.”

But the audience now is as much the Democratic superdelegates, who are especially attuned to politics and questions of electability in the fall, as it is rank-and-file voters.

Mrs. Clinton’s advisers said they had spent recent days making the case to wavering superdelegates that Mr. Obama’s association with Mr. Wright would doom their party in the general election.

That argument could be Mrs. Clinton’s last hope for winning this contest.