Obama picked up the backing of nine superdelegates, including Rep. Donald Payne of New Jersey, a member of the Congressional Black Caucus who had been a Clinton supporter.
In addition, the American Federation of Government Employees announced its support for Obama. The union claims about 600,000 members who work in the federal and Washington, D.C., governments.
Obama, who won a convincing victory in the North Carolina primary and lost Indiana narrowly on Tuesday, has been steadily gaining strength in the days since.
Clinton also gained a superdelegate.
The developments left the former first lady with 271.5 superdelegates, to 271 for Obama. Little more than four months ago, on the eve of the primary season, she held a lead of 169-63.
However, in a count conducted by The New York Times with CBS News, Obama leads Clinton, 266 to 263, the Times reported Friday on its Web site. Another survey -- by ABC New -- showed Clinton ahead as of Friday morning, the Times said.
Superdelegates are party leaders who attend the convention delegates by virtue of their positions, and are not selected in primaries and caucuses.
In addition to Payne, Reps. Peter DeFazio of Oregon and Mazie Hirono of Hawaii, two members of the Democratic National Committee from California and a party official in South Carolina announced they were supporting Obama. Superdelegates from New Mexico and Virginia also joined the migration.
So, too, John Gage, president of the AFGE.
"Our people, I think, recognize the enthusiasm and vitality behind Senator Obama's campaign," he said in a statement.
"The election is over, everybody knows that. Obama has won," said Vernon Watkins, one of the two Californians.
"After careful consideration, I have reached the conclusion that Barack Obama can best bring about the change that our country so desperately wants and needs," said Payne, who in a statement said that Clinton is a good friend and he still holds her in high regard.
Payne is one of at least 10 superdelegates who have switched allegiances from Clinton to Obama. None have publicly switched the other way.
In the overall race for the nomination, Obama leads with 1,859.5 delegates, to 1,697 for Clinton. Obama is just 165.5 delegates short of the 2,025 delegates needed to win it.
Clinton's new supporter was Rep. Chris Carney, D-Pa. His congressional district voted overwhelmingly for the former first lady in the Pennsylvania primary on April 22.
Both Obama and Clinton have courted superdelegates in recent days in private meetings at party headquarters not far from the Capitol.
Despite Watkins' assessment, Clinton has shown no signs she is ready to quit the race. She is heavily favored to win Tuesday's primary in West Virginia, and is in the midst of a two-day swing through several other states with upcoming elections.
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