Barack Obama and Hillary Clinton unveiled new negative television ads and attacked each other personally from the stump this weekend ahead of Tuesday's Pennsylvania primary, stoking more worries among Democrats that the party's eventual nominee will head into the general election badly damaged.

The rising vitriol is prompting more Democrats to demand that party leaders do something to end the battle. But no single leader or clique exists within the fractious party to end the fight, and those with influence insist voters must...