Lewis, a Democratic congressman from Atlanta, is the most prominent black leader to defect from Clinton's campaign in the face of near-unanimous black support for Obama in recent voting. He also is a superdelegate who gets a vote at this summer's national convention in Denver.
In a written statement, Lewis said Obama's campaign "represents the beginning of a new movement in American political history" and that he wants "to be on the side of the people."
"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 ... to express the will of the people," the statement said.
Lewis' endorsement had been a coveted prize among the Democratic candidates thanks to his standing as one of the most prominent civil rights leaders of the 1960s.
"John Lewis is an American hero and a giant of the civil rights movement, and I am deeply honored to have his support," Obama said in a statement. Obama said Wednesday night that he had yet to speak with Lewis about his decision. "I think it's very nice. It's appreciated," he said of Lewis' support.
'Under tremendous pressure'
Clinton, questioned about Lewis during a satellite interview with Houston television station KTRK, said: "I understand he's been under tremendous pressure. He's been my friend. He will always be my friend. At the end of the day it's not about who is supporting us, it's about what we're presenting, what our positions are, what our experiences and qualifications are and I think that voters are going to decide."
Lewis first announced his Clinton endorsement in October and has appeared on her behalf on television and at events across the country, at one point accusing Obama supporters of trying to fan the flames of race against her. Clinton frequently has cited his support in trying to establish her credentials among minority voters, saying she saw her campaign as a continuation of his work.
But Lewis came under intense pressure to get behind Obama after his constituents supported the Illinois senator roughly 3-to-1 in Georgia's Feb. 5 primary, and about 90 percent of black voters statewide voted for Obama, according to exit polls. The support among black voters nationwide to Obama's candidacy mirrors Lewis' Georgia district.
His change of heart follows a similar move by Rep. David Scott, a black Democrat who represents a neighboring district. It also comes a week after the Rev. Markel Hutchins, a young Atlanta minister, announced he would challenge Lewis in the Democratic congressional primary this summer.
Hutchins, 30, has seized on Lewis' waffling in the presidential contest as evidence that the 68-year-old congressman is out of touch.
"Today's announcement by Representative Lewis was clearly prompted by political expediency," Hutchins said Wednesday. "It is time for a change. It is time to send somebody to Congress who is actually willing to represent the district."
Call to Clinton
Earlier this month, Lewis' office disputed media reports that he said he would switch candidates, or was at least reconsidering. But until Wednesday, Lewis refused to answer questions clarifying his position.
He said Wednesday afternoon he had called former President Clinton and Sen. Clinton but had not reached them.
Lewis' announcement came the same day another superdelegate, Sen. Byron Dorgan of North Dakota, endorsed Obama, citing the presidential hopeful's record on trade.
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 grass-roots 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.