Senator Barack Obama has made significant inroads over the last month among the Democratic elected officials and party leaders known as superdelegates who will cast a fifth of the votes at the party’s convention, cutting into Senator Hillary Rodham Clinton’s long-held advantage with the group.

What began for Mr. Obama as a trickle of support from the superdelegates has grown into more of a stream as he has won the last 11 nominating contests in a row against Mrs. Clinton, of New York. Those victories have led to a few prominent defections  Representative John Lewis of Georgia formally switched sides this week  and prompted other undecided superdelegates to get off the fence and to support Mr. Obama. At least nine superdelegates have declared their support for Mr. Obama in the last few days.

The effect has been to whittle away at Mrs. Clinton’s lead with a group that her campaign had been counting on as a bulwark against the nomination’s going to Mr. Obama, of Illinois.

The Clinton campaign said Thursday that it had the support of 258 of the 795 superdelegates (not counting those from Florida or Michigan, whose delegations are the focus of a dispute), while the Obama campaign said it had the support of more than 200. It said it had won the support of 39 since Feb. 5, including four that formerly supported Mrs. Clinton.