The superdelegates are especially important this year because both campaigns believe that they could prove decisive in an election in which neither candidate is likely to win the 2,025 delegates needed to secure the nomination by the time of the convention in August.

Mr. Obama’s momentum in the recent primaries and caucuses appears to have influenced some superdelegates.

Marianne T. Stevens, a vice chairwoman of the Maine Democratic Party, announced this week that she would be casting her vote as a superdelegate for Mr. Obama. Ms. Stevens said she was moved to support him in part because his recent victories led her to believe that he would be the party’s nominee.

“He’s won, what, 11 primary contests in a row?” she said in a telephone interview. “The momentum doesn’t hurt. So yeah, I think he’s going to unify the upcoming primaries, and I think he’s going to win them.”

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But as the switch by Mr. Lewis has shown, these superdelegates can change their minds. And the Clinton campaign is counting on strong showings next week in Ohio and Texas, as well as the other states yet to vote, to retain its advantage with the party regulars who make up the bulk of the superdelegates.

For now, though, almost all of those publicly changing their minds are gravitating toward Mr. Obama.

Christine Samuels, a member of the Democratic National Committee who is active with the New Jersey N.A.A.C.P., said she switched her support from Mrs. Clinton to Mr. Obama in part because she was upset by some remarks the Clintons made around the time of the South Carolina primary.

But Ms. Samuels also cited Mr. Obama’s recent successes in the nominating contests.

“I wouldn’t have switched if I didn’t think he could win,” she said.

Representative John Barrow, a Georgia Democrat who faced a close re-election battle in 2006, announced Thursday that he would support Mr. Obama. “Seventy percent of the folks in my district voted for Senator Obama,” Mr. Barrow said, “and I think they voted for him for some of the same reasons I think people voted for me.”

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As the Obama campaign has advanced the notion that the superdelegates should follow the popular vote, the Clinton campaign has countered that the superdelegates have a responsibility to do what they think is best for the Democratic Party and the nation. It even has a Web site — www.delegatehub.com — that notes none of the superdelegates are “required to cast a vote on the basis of anything other than his or her best judgment about who is the most qualified to be president.”

The Clinton campaign notes that it has its own defectors. On Thursday, it announced that Veronica Escobar, an El Paso County, Tex., commissioner, was switching her support from Mr. Obama to Mrs. Clinton.

Unfortunately for the Clinton campaign, Ms. Escobar is not a superdelegate.