Several polls signal that Mr. Obama’s troubles have shaken the confidence in the Democratic electorate, and it is too early to measure fully how much he may have been hurt by a round of incendiary appearances by his former pastor, the Rev. Jeremiah A. Wright Jr.

A big Clinton victory in Indiana on Tuesday and in West Virginia the next week could, combined with her victories in Pennsylvania and Ohio, give her ammunition to say that Mr. Obama would fail to draw blue-collar support against Mr. McCain in the fall.

David Plouffe, the manager of Mr. Obama’s campaign, said that if Mrs. Clinton won 55 percent of the remaining pledged delegates  an assumption he called “overly generous”  she would still need about two-thirds of the remaining uncommitted superdelegates to reach the 2,025 delegates needed to secure the nomination.

Mrs. Clinton’s advisers did not dispute Mr. Plouffe’s calculation, in effect acknowledging the enormousness of their task.

Senator Christopher J. Dodd, the Connecticut Democrat who ran for president and has now endorsed Mr. Obama, said that Mrs. Clinton still had a chance but that it was something of a long shot. “I still don’t think the math is there,” Mr. Dodd said.

Most of what chance Mrs. Clinton has rests on her ability to convince superdelegates that Mr. Obama would be a flawed candidate in the general election. In recent weeks, he has certainly struggled with a series of problems  some of his own making  which has made it easier for her to make that case.

Many superdelegates said they were queasy about Mr. Obama and his former pastor, and fearful of how the issue might be used in the fall. Still, they said they were not convinced that that made him a weaker general-election candidate than Mrs. Clinton, or at least not convinced enough to cast a vote that could be portrayed as overturning the will of Democratic primary voters and blocking the effort by Mr. Obama to become the nation’s first African-American president.