Clinton stepped before her own supporters not long afterward in Indianapolis. "Thanks to you, it's full speed on to the White House," she said, signaling her determination to fight on in a campaign already waged across more than 16 months and nearly all 50 states.
Returns from 99 percent of North Carolina precincts showed Obama winning 56 percent of the vote to 42 percent for Clinton, a triumph that mirrored his earlier wins in Southern states with large black populations.
That made Indiana a virtual must-win Midwestern contest for the former first lady, who had hoped to counter Obama's persistent delegate advantage with a strong run through the late primaries.
Returns from 99 percent of the precincts showed her with 51 percent to 49 percent for her rival, a margin of little more than 22,000 votes out of more than 1.2 million cast. The outcome wasn't clear for more than six hours after the polls closed, the uncertainty stemming from slow counting in Lake County near Obama's home city of Chicago.
Obama won at least 69 delegates and Clinton at least 63 in the two states combined, with 55 still to be awarded.
Voters in both states fell along racial lines long since established in a marathon race between the nation's strongest-ever black presidential candidate and its most formidable female challenger for the White House.
The economy was the top issue by far in both states, according to interviews with voters as they left their polling places.
Two weeks after a decisive defeat in Pennsylvania, Obama sounded increasingly like he was looking forward to the fall campaign.
"This primary season may not be over, but when it is, we will have to remember who we are as Democrats ... because we all agree that at this defining moment in history - a moment when we're facing two wars, an economy in turmoil, a planet in peril - we can't afford to give John McCain the chance to serve out George Bush's third term."
Clinton was joined at her rally by her husband Bill, his face sunburned after hours spent campaigning in small-town North Carolina, and their daughter, Chelsea.
She stressed the issue that came to dominate the final days of the primaries in both states, her call for a summertime suspension of the federal gasoline tax. "I think it's time to give Americans a break this summer," she said.
She added that no matter who wins the epic race for the nomination, "I will work for the nominee of this party" in the fall campaign against the Republicans. To emphasize her determination, Clinton announced plans to campaign Thursday in West Virginia, South Dakota and Oregon, three of the remaining primary states.
Obama was gaining more than 90 percent of the black vote in Indiana, while Clinton was winning an estimated 61 percent of the white vote there.
In North Carolina, Clinton won 60 percent of the white vote, while Obama claimed support from roughly 90 percent of the blacks who cast ballots.
Obama's delegate haul edged him closer to his prize - 1815.5 to 1,672 for Clinton in The Associated Press count, out of 2,025 needed to win the nomination.
As he told his supporters, Obama was on pace to finish the night within 200 delegates of the total needed. There are 217 delegates at stake in the six primaries yet to come. Another 270 superdelegates remain uncommitted.
He has long led Clinton among delegates won in the primaries and caucuses, and has increasingly narrowed his deficit among superdelegates who will attend the convention by virtue of their status as party leaders. The AP tally showed Clinton with 269.5 superdelegates, and Obama with 255.
The impact of a long-running controversy over Obama's former pastor, the Rev. Jeremiah Wright, was difficult to measure.
In North Carolina, six in 10 voters who said Wright's incendiary comments affected their votes sided with Clinton. A somewhat larger percentage of voters who said the pastor's remarks did not matter supported Obama.
The questionnaire used to learn about voter motivation did not include any questions about the gasoline tax.
In Indiana, about one in five voters said they were independents, an additional one in 10 said Republican. Only Democrats and unaffiliated voters were permitted to vote in North Carolina.
Voting in Indiana was carried out under a state law, recently upheld by the Supreme Court, that requires voters to produce a valid photo ID. About a dozen nuns in their 80s and 90s at St. Mary's Convent in South Bend were denied ballots because they lacked the necessary identification.
Obama leads Clinton in delegates won in primaries and caucuses. Despite his defeat two weeks ago, he has steadily whittled away at her advantage in superdelegates in the past two weeks and trails 269.5 to 255.
Clinton saved her candidacy with her win in Pennsylvania, and she campaigned aggressively in Indiana in hopes of denying Obama a victory next door to his home state of Illinois. Indiana is home to large numbers of blue-collar workers who have been attracted to the former first lady, and she sought to use her call for a federal gas tax holiday to draw them and other economically pinched voters closer.
Inevitably, the issue quickly took on larger dimensions.
Obama said it symbolized a candidacy consisting of "phony ideas, calculated to win elections instead of actually solving problems."
Clinton retorted, "Instead of attacking the problem, he's attacking my solutions," and ran an ad in the campaign's final hours that said she "gets it."
The balance of the primary schedule includes West Virginia, with 28 delegates on May 13; Oregon with 52 and Kentucky with 51 a week later; Puerto Rico with 55 delegates on June 1, and Montana with 16 and South Dakota with 15 on June 3.
Sen. McCain of Arizona, the Republican nomination already in hand, campaigned in North Carolina and assailed Obama for his vote against confirmation of Chief Justice John Roberts.
"Senator Obama in particular likes to talk up his background as a lecturer on law, and also as someone who can work across the aisle to get things done," McCain said. "But ... he went right along with the partisan crowd, and was among the 22 senators to vote against this highly qualified nominee."
Clinton also voted against Roberts, but McCain, as is often the case, focused his remarks on Obama.
Obama's campaign responded that the Republican would pick judges who represent a threat to abortion rights and to McCain's own legislation to limit the role of money in political campaigns.
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David Espo reported from Washington. Associated Press writers Tom Raum in Indianapolis and Libby Quaid in Winston-Salem contributed to this report.
