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.
"I'm gratified that we've got some superdelegates who are coming our way. And I think we've got a strong case to make that I will be a nominee that can pull the party together and take on John McCain. Our focus has always been on the pledged delegates and just getting the American people to vote for us. And we think that ultimately that should be the strongest measure of who's the nominee," Obama told reporters in Woodburn, Ore.
Clinton also gained two superdelegates.
The developments left the former first lady with 272.5 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.
Superdelegates are party leaders who attend the convention delegates by virtue of their positions, and are not selected in primaries and caucuses.
In an interview with National Public Radio, former candidate John Edwards said Clinton has made a compelling case for her candidacy, but "I think it's very hard for her now to make a compelling case for the math. I mean, I think that's the reality of what she's faced with. She knows that. ... It's just very hard to see how the math works."
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 1,698 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.
