Obama widened his delegate lead on the biggest day left on the primary calendar, though it appeared that the race could still continue through the final contests, less than four weeks from now.
Obama claimed victory in North Carolina and congratulated Clinton on apparently winning Indiana, where incomplete returns gave her a narrow lead.
But Clinton is running out of time and delegate contests as she attempts to stop Obama, who appears increasingly likely to become the first African-American nominee of a major party.
Yesterday, Obama reversed a two-month slide that had raised increasing questions about his chances to defeat John McCain in November.
By running up a large victory in North Carolina, the nation's 10th-most-populous state, he countered Clinton's argument that he cannot win the large states. He also padded his popular-vote advantage, making it more difficult for her to win the total vote when the primaries end.
"Tonight we stand less than 200 delegates away from securing the Democratic nomination," Obama told an election-night rally crowd in Raleigh, N.C.
Clinton appeared more subdued when she appeared later in the evening before supporters in Indiana, the state Obama called "the tiebreaker," between Pennsylvania, where she won last month, and North Carolina, where he was favored.
Clinton alluded to Obama's remark and claimed victory in Indiana before the final returns were in and with the outcome still in doubt.
"Thanks to you, it's full speed on to the White House," said Clinton, vowing to go on while also delivering a televised plea for fresh donations to assist her cash-short campaign.
Clinton passed up appearances on the network morning shows today and returned to Washington with no public events on her schedule. There was inevitable speculation about whether her candidacy was nearing an end, but her campaign announced that she would fly to West Virginia, South Dakota and Oregon for ralliestomorrow.
In their election night remarks, both candidates sent mixed signals about the next phase of the campaign, suggesting that the prolonged nomination contest may be nearing a resolution and that it might not extend beyond the next few weeks.
Clinton said it was "on to West Virginia, Kentucky, Oregon and the other states where people are eager to have their voices heard. She also insisted that the votes of Michigan and Florida be counted.
"But I can assure you," she also said, "no matter what happens, I will work for the nominee of the Democratic Party, because we must win in November."
Clinton faces a steeper challenge than perhaps at any time in the long campaign.
She had boasted last Friday that winning North Carolina would be "a game-changer," a claim that Obama highlighted in his victory speech. But in a sign that the most confrontational portion of the campaign is past, he also dropped a line from his prepared text that said Americans weren't looking for "more gimmicks," a dig at Clinton's gas-tax holiday proposal, which may well have worked against her and helped Obama, who strongly criticized it.
Clinton's double-digit loss in North Carolina likely dims, perhaps considerably, her chances of persuading Democratic superdelegates that Obama's candidacy had been irrevocably weakened by recent controversies over his comments about small-town residents and remarks by his former pastor, the Rev. Jeremiah A. Wright Jr.
The furor over Wright did not seem to be a decisive issue, exit polls showed, though white voters in North Carolina who called it important favored Clinton by better than a 5-1 margin.
Interviews with voters as they left their polling places showed that Obama had made modest gains in Indiana among white voters, including women and Catholics, groups he lost by 30 percentage points or more in neighboring Ohio in early March. In Indiana, Obama lost those groups by about 20 points, according to exit polling for the Associated Press and the TV networks.
Clinton did well enough among white voters, particularly those with less than a college education and with family incomes under $75,000, to make the Indiana contest close.
In North Carolina, demographics favored Obama. Black voters made up about one-third of the electorate and went for him by better than 10-1, repeating a pattern that was established in next-door South Carolina in January, when former President Bill Clinton made comments that angered black voters. Clinton won 60 percent of the white vote in North Carolina, not enough to overcome Obama's advantage among African Americans.
First-time primary voters cast about one-fifth of the ballots in each state and went for Obama by landslide margins, accounting for roughly half of his North Carolina victory margin.
In Indiana, Obama won among self-described independents, a group that has favored him throughout the campaign, but narrowly lost that vote to Clinton in North Carolina.
Clinton won white voters over 45 by wide margins in both Indiana and North Carolina, as she has in other states, though Obama narrowed her advantage among those voters, exit polls showed, and outperformed her among under-30 whites.
Voter turnout set records in both states, which had not held meaningful Democratic presidential primaries in decades.
Amid high gasoline prices and Clinton's hotly debated proposal for a gas-tax holiday, the economy was cited by roughly two in every three of primary voters as the most important issue, the highest proportion of the '08 campaign. The war in Iraq was the top concern for only one voter in five.
With six primaries remaining, Clinton could well collect more delegates than Obama in the final contests. She is favored in West Virginia next week, Kentucky on May 20 and Puerto Rico on June 1.
But the relatively small number of delegates at stake and party rules that award almost as many delegates to the runner-up mean she has no chance of overtaking Obama in pledged delegates by the time the primaries end.
Obama's camp projects that he will gain a majority of the pledged delegates when Oregon votes May 20, a contest he is expected to win.
Neither candidate can secure the nomination with pledged delegates alone, putting the power to choose a nominee in the hands of about 800 superdelegates, elected and party officials who are free to vote for either candidate.
Some 220 superdelegates remain undeclared, and Clinton would need considerably more of them than Obama in order to prevail, according to current projections. The Obama campaign estimates that she would need to win 70 percent of the remaining superdelegates to become the nominee.
Clinton's campaign is hoping that party leaders will help her close the pledged delegate gap by resolving a long-running dispute over awarding delegates from Florida and Michigan, whose primaries did not count toward the nomination because they were held in violation of party rules. On May 31, the Rules and Bylaws Committee of the Democratic National Committee will meet in an effort to resolve the impasse.
A plan the panel is considering would award half of Michigan and Florida's delegates on the basis of the primary vote, giving Clinton about 55 more delegates than Obama. That would still not be enough for her to overtake him in pledged delegates.
With neither state included, it would take 2,025 to clinch the nomination. Going into this week's primaries, Obama had 1,745.5 delegates, to 1,608 for Clinton, according to the Associated Press.