The former first lady said in Philadelphia after her victory that the result proved the contest was swinging in her favour.
A count by MSNBC news showed Clinton gained nine delegates on Obama following the Pennsylvania victory.
Obama now has 1,726 delegates to Clinton's 1,593, with 2,024 needed to clinch the nomination, it said.
Clinton has sought to persuade the 795 superdelegates, senior Democratic officials who can vote for either candidate, that she alone is capable of beating John McCain, the presumptive Republican candidate, in November's presidential poll.
"At the end of the day, people have to decide who they think would be not only the best president, which is the most important question, but who would be the better candidate against Senator McCain," she told NBC.
Clinton, who needed a victory to boost donations to her campaign, said she had raised about three million dollars online since Tuesday night.
Both candidates were now looking ahead to the next round of primary votes on May 6 in North Carolina, where Obama is set to win, and Indiana, where a close race is expected.
However, Obama's campaign dismissed the former first lady's chances of making a significant dent in the Illinois senator's delegate lead.
Obama is "less than 300 delegates away from securing the nomination," said David Plouffe, his campaign manager.
"Senator Clinton would need 70 per cent of all the remaining pledged delegates to erase the lead in pledged delegates."
Clinton's win renewed the prospect that the battle for the nomination could drag on to the Democratic convention in Colorado in August, despite the hopes of party leaders to avoid a drawn-out battle.
As Pennsylvanians went to the polls, Clinton issued a threat to "obliterate" Iran if it launched a nuclear attack against Israel, a move Obama called "sabre rattling".
Tuesday's vote saw women and older voters, groups that have tended to support Clinton in previous states, come out in force.
Interviews with voters leaving the polls showed almost six-in-10 were women and three-in-10 were age 65 or over.
One in five voters said they decided who to vote for within the last week and about one in 10 said they made up their minds only on Tuesday, US media exit polls said.
But exit polls showed Clinton won about 58 per cent of those who decided in the last week.
A quarter of voters had household family income of more than $100,000 last year, and about as many reported having a postgraduate degree - both groups have tended to vote for Obama.