Hillary Rodham Clinton has edged ahead in the Pennsylvania primary in a fight for survival against Barack Obama. She needs a big win to allow her to stay in the race for the Democratic presidential nomination and keep alive her once-high hopes for winning the White House. Early precinct votes from across the state Tuesday night showed Clinton running ahead in the western part of the state, including Pittsburgh, as well as the area around Scranton in the east with large numbers of working-class voters. Obama was ahead in Philadelphia, the biggest city, and the populous surrounding suburbs. Clinton was heavily favored to win Tuesday in Pennsylvania, the largest prize remaining in the primary season and a state dominated by voters who have flocked to her in the past.