Lewis, a Democratic congressman from Atlanta, is the most prominent black leader to defect from Clinton's campaign in the face of near-majority black support for Obama in recent voting. He also is a superdelegate who gets a vote at this summer's national convention in Denver.
"After taking some time for serious reflection on this issue, I have decided that when I cast my vote as a superdelegate at the Democratic convention, it is my duty as a representative of the 5th Congressional District to express the will of the people," Lewis said in a statement. "As a U.S. representative, it is my role not to try to subdue or suppress the will of the people, but to help it prosper and grow."
Lewis' constituents supported Obama roughly 3-to-1 in Georgia's Feb. 5 primary. His endorsement had been a coveted prize among the Democratic candidates thanks to his standing as one of the last major civil rights leaders of the 1960s.
Lewis said he had tried to contact former President Clinton and Hillary Clinton.
"I think the candidacy of Senator Obama represents the beginning of a new movement in American political history that began in the hearts and minds of the people of this nation," he said. "And I want to be on the side of the people, on the side of the spirit of history."
The reservations Lewis is now expressing follow endorsements for Obama from two U.S. senators who'd been on the fence: North Dakota's Byron Dorgan today and Connecticut's Chris Dodd yesterday, reports CBS News correspondent Jim Axelrod. The Democratic Party establishment seems like it's trying to tell Clinton something-that it may be time to get out of the race.
The Obama campaign also said more than 1 million people have contributed to the campaign - a threshold crossed on Wednesday. Many donors have given $25 or $50, he said. The average donation is a little more than $100.
"We have funded this campaign at the grassroots level," campaign manager David Plouffe told reporters on a conference call. "It's really built on the backs of the American people who are getting involved in this campaign. Most of the people giving us money are also volunteering."
Dorgan said Obama has supported key trade issues. "He and I feel the same way. We both believe in trade and plenty of it. We just insist it that it be fair to our country - the rules be fair."
NAFTA, the free trade agreement with Canada and Mexico, is unpopular with blue-collar workers whose votes are critical in the Democratic primary Tuesday in Ohio.
Obama has won 11 straight primaries and caucuses since Super Tuesday, increased his advantage in the all-important delegate count and has attracted the support of his congressional colleagues. On Tuesday, he secured the endorsement of one-time presidential candidate Sen. Chris Dodd of Connecticut.
Clinton has been endorsed by 13 of her Senate colleagues, Obama 10.
Dorgan was an ally of former President Clinton and a vocal critic of President Bush. As chairman of the Democratic Policy Committee, he has led hearings on government accountability issues related to the Iraq war and hurricanes on the Gulf Coast.
Dorgan has built a reputation for championing populist farm programs, criticizing Republican free-trade policies and assailing big business. He made headlines in 2005 when he called for a windfall profits tax on major oil companies.
Last year, he authored a measured to block funding of a Department of Transportation pilot program required under NAFTA that would have opened the U.S. to cross-border long-haul Mexican tractor trailers. The program was opposed by the Teamsters Union, among others.