Democratic hopeful Hillary Clinton has vowed to continue her campaign despite losing ground in the latest primary contests and her financial problems.
Mrs Clinton beat rival Barack Obama by just two points in Indiana's primary, while he won by 14 in North Carolina.
The votes were the final major Democratic primaries which help decide the party's White House candidate.
Mrs Clinton is also facing a funding crisis - she was forced to loan her campaign $6.4m (£3.3m) last month.
Unhealthy sign
Speaking to reporters, she pledged to remain in the race "until the nominee is chosen", although she did not reveal whether this meant she would stay in until the party's nominating convention in August.
Mrs Clinton also appealed to supporters for more funds to carry on the fight.
That is not a healthy sign, says the BBC's Justin Webb in West Virginia, where Mrs Clinton is campaigning.
Contributors will not throw money at what they see as a lost cause, and without daily infusions of cash, presidential campaigns cannot survive, our correspondent says.
The two candidates are locked in a drawn-out battle to stand for the Democratic Party against Republican John McCain in November's presidential election.
With neither likely to win enough pledged delegates to the party conference in August to clinch the nomination, the outcome hangs on the decision of nearly 800 so-called super-delegates - senior party members - who can choose which candidate to back.
The Clinton campaign is hoping that it can still persuade a majority of the remaining undecided super-delegates to back the New York senator.
Mrs Clinton met some of them on the day after her North Carolina defeat in an effort to shore up her support.
So far she has received endorsements from 271 superdelegates, to Mr Obama's 256, with 270 still undecided, according to the Associated Press.
Many super-delegates say they will vote for the candidate chosen in the primary of their home state.
But the campaign suffered a setback when former senator and 1972 Democratic presidential candidate George McGovern announced that he was switching his support from Mrs Clinton to Mr Obama.
Mr McGovern, who is not a super-delegate and will therefore not have a vote at the party's nominating convention, also called on Mrs Clinton to withdraw from the race.
Her team are also still arguing that the delegates from Michigan and Florida - barred by party chiefs from attending the convention after the states broke party rules by holding their contests early - should be allowed to take their seats at the convention after all.
Mrs Clinton did well in the disputed contests, and would benefit if the party opted to overturn its earlier ruling.
Mr Obama is leading the race in delegates who will choose the presidential nominee by 1,840 to 1,684, according to the Associated Press news agency.
Race split
Speaking to a raucous rally in Raleigh, North Carolina, on Tuesday night, Mr Obama said: "Tonight we stand less than 200 delegates away from securing the Democratic nomination for president of the United States."
Analysts said Mr Obama's win was vital after a recent difficult campaign stretch.
He has been dogged by controversy over his gaffe that small-town residents were "bitter", and racially charged comments by his former pastor, Reverend Jeremiah Wright.
A win in Indiana was seen as critical for Mrs Clinton to stay in the race to stand against Republican John McCain in November's presidential vote.
At her election gathering in the Indiana state capital, Indianapolis, she told cheering supporters it was "full speed on to the White House".
But despite her defiant words, Mrs Clinton seems to have lost some of her trademark fighting spirit, says BBC Washington correspondent Jonathan Beale.
The rivals, courting voters suffering from an ailing economy, spent recent days sparring over Mrs Clinton's proposal to suspend the federal petrol tax for the summer.
Mid-western Indiana is home to large numbers of blue-collar workers, a bloc which has backed Mrs Clinton in previous contests.
According to exit polls conducted for AP, about two-thirds of white voters in both states who had not completed a college education supported Mrs Clinton.
In North Carolina, Mr Obama won the backing of 90% of the state's African-American voters, who make up more than a third of the state's electorate.
Mrs Clinton won 58% of non-black voters in the state, according to the polls.
There are just six primary contests left: West Virginia, Oregon, Kentucky, Puerto Rico, Montana and South Dakota.
The Republicans also held primaries in Indiana and North Carolina on Tuesday, but the votes were largely symbolic as Senator John McCain has effectively secured the nomination.