After the final primaries, on June 3, either candidate would have to get votes from the roughly 300 undeclared superdelegates to win the nomination, with Clinton needing more than Obama.
The impact of the bruising Pennsylvania battle, which ended in a flurry of attack ads by both sides, could be seen in an opinion survey of voters as they left their polling places.
Obama's widely quoted gaffe, that small-town Pennsylvania voters cling to guns and religion, appeared to have hurt him among church-goers. They turned out in larger proportions than in last month's Ohio primary and went strongly for Clinton.
According to exit polling conducted for the Associated Press and television networks, Clinton carried the votes of Democrats who attend church weekly by nearly 20 percentage points. In Ohio, her margin among that group was just 4 points.
Obama also lost among voters from gun-owning households, who cast more than 1 in 3 primary votes and favored Clinton by a margin of 62 percent to 38 percent.
Reflecting the negative tenor of the contest, voters were more critical of the way both candidates ran their campaigns than were voters in next-door Ohio last month. But substantially more voters said Clinton, rather than Obama, attacked unfairly.
Obama strategists had explained his 10-point loss to Clinton in Ohio by saying that voters had not had enough time to get to know him well. But among several key groups in Pennsylvania - especially blue-collar whites - he again ran poorly.
Clinton was helped by a strong vote from Catholics (who favored her by more than 2-to-1), white women nearly 2-to-1), blue-collar whites, (who supported her by a margin of 66 percent to 34 percent) and seniors. She won voters ages 65 and over by 21 points, down from 45 in Ohio.
Obama's campaign effort to register new voters helped him cut into Clinton's lead, as did the state's relatively larger (compared with Ohio) proportion of voters with postgraduate degrees, who favored Obama by a small margin.
Obama also won more than 90 percent of the black vote, in spite of Clinton's active support from Philadelphia Mayor Michael Nutter, an African-American. Blacks cast 13 percent of the total vote, down from 18 percent in Ohio.
Earlier in the day, Obama tried to address the concerns of some in his party that a significant number of Clinton supporters would defect to McCain if Obama is the Democratic nominee.
