His triumph seemed unlikely to shorten a Democratic marathon expected to last at least six more weeks - and possibly far longer - while Republicans and their nominee-in-waiting, Sen. John McCain, turn their attention to the fall campaign.
"Now we look forward to campaigning in Pennsylvania and around the country," Maggie Williams, Clinton's campaign manager, said in a written statement that congratulated Obama on his victory.
"I'm confident that once we get a nominee, the party is going to be unified," Obama said as he collected his victory.
But in a race growing more contentious, he took a swipe at the way his rival's campaign has conducted itself.
"We've been very measured in terms of how we talk about Senator Clinton," he said. "I've been careful to say that I think Senator Clinton is a capable person and that should she win the nomination, obviously, I would support her. I'm not sure we've been getting that same approach from the Clinton campaign," he said in on CNN.
Returns from 92 percent of Mississippi's precincts showed Obama gaining 59 percent, to 39 percent for Clinton.
Obama picked up at least 17 of Mississippi's 33 delegates to the Democratic National Convention, with five more to be awarded. He hoped for a win sizable enough to erase most if not all of Clinton's 11-delegate gain from last week, when she won three primaries.
The Illinois senator had 1,596 delegates to 1,484 for Clinton. It takes 2,025 to win the nomination.
Neither of the two rivals appears able to win enough delegates through primaries and caucuses to prevail in their historic race for the nomination, a development that has elevated the importance of nearly 800 elected officials and party leaders who will attend next summer's national convention as unelected superdelegates.
Obama leads Clinton among pledged delegates, 1,385-1,237 in The Associated Press count, while the former first lady has an advantage among superdelegates, 247-211.
There was little suspense about the Mississippi outcome, and both Clinton and Obama spent part of their day campaigning in Pennsylvania, which has 158 delegates at stake in a primary on April 22.
The volatile issue of race has been a constant presence in the historic Democratic campaign, and it resurfaced during the day in the form of comments by Geraldine Ferraro, the 1984 Democratic vice presidential candidate and a Clinton supporter.
"If Obama was a white man, he would not be in this position. And if he was a woman (of any color) he would not be in this position. He happens to be very lucky to be who he is. And the country is caught up in the concept," she said in an interview with the Daily Breeze of Torrance, Calif., that was published last Friday.
Clinton expressed disagreement with Ferraro's comments, and said, "It's regrettable that any of our supporters - on both sides, because we both have this experience - say things that kind of veer off into the personal."
Obama called Ferraro's remarks "patently absurd."
Blacks, who have supported Obama in overwhelming numbers in earlier primaries, accounted for roughly half the ballots cast in Mississippi, according to interviews with voters leaving polling places.
About one in six Democratic primary voters were independents, and Clinton and Obama split their support. Another 10 percent of voters were Republican, and they preferred Clinton by a margin of 3-1.
Six in 10 Obama supporters said he should pick the former first lady as his vice presidential running mate if he wins the presidential nomination. A smaller share of Clinton's voters, four in 10, said she should place him on the ticket.
The Republican primary provided even less suspense than the Democratic contest. McCain had already amassed enough delegates to win his party's nomination and was in New York, attending an evening fundraiser that was expected to raise $1 million.
Adding to the uncertainty in the lengthening race between Obama and Clinton, Democrats from Florida and Michigan are pressing for their delegations to be seated at the summer convention.
Both states were stripped of their delegates by the Democratic National Committee after they held early primaries in defiance of party rules. Efforts are under way to find a compromise that would satisfy party leaders in both states as well as the candidates, although Obama and his top strategist were cool during the day to proposals for primaries-by-mail. "I think there are some concerns in terms of making sure that whatever we do is fair and votes are properly counted and the logistics make sense," Obama told CNN.
Obama has defeated Clinton in primaries in South Carolina, Alabama, Georgia and Louisiana, other states where blacks cast a large share of the ballots.
Exit polls showed blacks accounted for a majority of the ballots in all but Louisiana, where they represented a plurality. Obama's share of the black vote in those states ranged from 78 percent in South Carolina to 88 percent in Georgia, while Clinton won the white vote with ease.
After losing 12 straight primaries and caucuses, Clinton rebounded smartly last week with primary victories in Ohio, Texas and Rhode Island. Obama won the Vermont primary, led in the Texas caucuses, and suffered a loss of only 11 delegates.
But the damage was deeper than mere numbers - costing him a chance to rally uncommitted party leaders to his side, and depriving him of an opportunity to drive the former first lady from the race.
Reinvigorated, Clinton immediately began talking about the possibility of having Obama as her running mate.
Obama ridiculed the idea, saying, "I don't know how somebody who is in second place is offering the vice presidency to the person who is first place."
Other than Pennsylvania, Indiana, North Carolina, West Virginia, Kentucky, Oregon, Puerto Rico, Montana and South Dakota have primaries remaining.
