He picked up at least six Mississippi delegates to the Democratic National Convention, with 27 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.
Obama began the night with 1,579 delegates, to 1,473 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, 1,368-1,226 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.
Nearly one in five Democratic primary voters called himself an independent. About one in 10 was Republican.
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. Sen. John McCain or Arizona 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. But efforts are under way to find a compromise that would satisfy party leaders in both states as well as the candidates, possibly through primaries-by-mail.
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.
Both Obama and Clinton campaigned in Mississippi, although the former first lady seemed to go out of her way to say she did not expect to win.
"Some people have said 'Well Mississippi is very much a state that will most likely be in favor of Senator Obama.' I said 'Well, that's fine,' but I want people in Mississippi to know I'm for you," she said in Hattiesburg before flying to Pennsylvania.
Obama made a stop Tuesday in Greenville before heading to Pennsylvania, too.
"I've been praying for you," a man called out.
"I believe in prayer," Obama replied.
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."
House Speaker Nancy Pelosi of California, who hasn't endorsed either candidate, said a unity ticket was impossible.
She said the Clinton campaign "has fairly ruled that out by proclaiming that Senator McCain would be a better commander in chief than Obama."
Other than Pennsylvania, Indiana, North Carolina, West Virginia, Kentucky, Oregon, Puerto Rico, Montana and South Dakota have primaries remaining.
*Are you a politics junkie? We have more political gems on our four political blogs written by a White House insider, PhD and Eyewitness News reporters.