Turnout was heavy at many polling places, and a record number of Pennsylvanians had registered to vote.
The result followed Clinton's popular vote victories in Ohio, California, New Jersey and Texas, fuelling her argument she is the Democrat who can capture the big and diverse states where the party will need to do well in November.
Obama, an Illinois senator who would be the first black U.S. president, won 9 of every 10 black voters and led among young and male voters. Clinton, who would be the first woman U.S. president, won blue-collar workers, elderly voters and more than half of women.
CLINTON WINS LATE DECIDERS
She also won 58 percent of those who decided in the last week, when Obama was on the defensive in a debate over a series of campaign controversies and Clinton questioned his toughness in an ad featuring images of al Qaeda leader Osama bin Laden.
Voters were not happy with the race's negative tone. About two-thirds of Pennsylvania voters thought Clinton unfairly attacked Obama, while about half thought Obama had unfairly attacked Clinton, the polls showed.
Obama has a nearly insurmountable lead in popular votes won during the first three months of the primary battle and in delegates. But neither candidate can clinch the nomination without the help of superdelegates - nearly 800 party insiders who are free to support either Obama or Clinton.
Clinton hopes her win ignites a strong run through the last nine contests, bringing her closer in delegates won and votes cast and convincing superdelegates she is the Democrat who can beat McCain.
The victory in Pennsylvania cut Obama's lead in votes won during the nomination battle to about 500,000. A partial delegate count in the state gave Clinton 75 delegates to Obama's 65, according to MSNBC.
The MSNBC count gave Obama 1,720 delegates overall to Clinton's 1,588, well short of the 2,024 needed to clinch the nomination.
Democratic rules allow the losers in each state to win a proportional amount of delegates, giving Obama big chunks of delegates even when he loses. That means Clinton must win many of the nine remaining contests by big margins to significantly close the gap with Obama in the delegate race.
