The latest polls showed the race was a dead heat with the outcome in the hand of voters who were still undecided.
One of those undecideds was Andrew Sitkoff, 56, a physician from East Bradford Township.
Speaking outside his polling place at the Chester County Art Association this morning, he said he did not make up his mind between Sestak and Specter until moments before voting.
He said he opted for Sestak because Specter's age was a concern. But Sitkoff said he also needed to be convinced that Sestak could beat the likely Republican candidate, Pat Toomey, in the fall.
Philadelphia's media market in the meantime buzzed with last minute advertising blitzes by Specter and Democratic gubernatorial hopeful Anthony H. Williams, a state senator from Philadelphia who has a well financed war chest.
Reports from around the city indicated a light turnout, which does not bode well for Specter or Williams.
"We need a large turnout in Philadelphia," Gov. Rendell, a supporter of both men, said in lamenting the low turnout.
Williams is one of four Democrats in the gubernatorial race.
Rain, which fell through the day, is forecast to back off by the evening peak voting period.
Turnout in the meantime also appeared flat or low in the Philadelphia suburbs.
There are 4.3 million Democrats and 3.1 million Republicans registered to vote in Pennsylvania.
An additional 1 million independents can cast a ballot only if there is a referendum in their area. There are four referendum questions on the ballot in Philadelphia.
Staff writers Kathleen Brady Shea, Bonnie Cook, John Martin, Mari Schaefer and Anthony R. Wood contributed to this story