A New York congresswoman is seeking to allay concerns that Democratic superdelegates would switch crucial votes to Senator Obama from Senator Clinton now that momentum in the presidential campaign appears to have shifted in Mr. Obama’s direction. Rep. Nydia Velazquez, who is among the 796 Democratic superdelegates who could be in a position to award the nomination at the party’s national convention, said her endorsement of Mrs. Clinton is final.

“I’ll be with Hillary until the end,” she told reporters yesterday at City Hall.

Ms. Velazquez, who represents parts of Brooklyn, Queens, and Manhattan, said concerns that party leaders may end up deciding the contest between Mr. Obama and Mrs. Clinton are being raised too late to consider a suggestion forwarded by some Democrats — that all delegates vote according to the popular vote.

“Well, we don’t want to change the rules now,” Ms. Velazquez said. “I think that if people want to change the rules of the game, you have to do it immediately, right when we start a campaign like this one.”

The role of superdelegates has drawn scrutiny as Mr. Obama and Mrs. Clinton compete for the support of pledged delegates. Some are concerned that superdelegates, comprising about 20% of the delegate total, could override a clear victory by one of the candidates in the primaries and draw the party into a bitter squabble over the winner’s legitimacy. According to CNN, Mrs. Clinton currently holds the support of at least 224 superdelegates versus 135 backing Mr. Obama.