Obama asked the Sestak supporters to turn out with the enthusiasm that Democrats showed when he was elected in 2008.
Republicans were ready for Obama's visit. Toomey, campaigning near Harrisburg, said the fundraiser would remind voters that Sestak has been "lockstep" with Obama's legislative agenda.
Rob Gleason, chairman of the state Republican Party, told reporters that the White House tried to "bribe" Sestak to drop out of Senate primary last year.
The White House had former President Bill Clinton offer Sestak a spot on a "senior executive branch advisory board" if he would drop a challenge to U.S. Sen. Arlen Specter, according to a White House Counsel memo released in May.
Sestak declined that offer and defeated Specter in May.
Obama was greeted by more than 100 city union workers from District Council 33 and District Council 47, chanting, "No contract, no peace" as the president ducked into the Reading Terminal Market for a quick cheesesteak and some chatter with stunned shoppers.
Pete Matthews, who heads DC 33, which represents the city's blue-collar workers, said the unions were there to put pressure on Nutter for a contract. He added that the unions and their retirees represent 90,000 city voters who will turn on Nutter in next year's primary election if they don't get a contract.
"We're going to fight back with our vote," Matthews told the crowd, raising last week's primary election defeat of Washington Mayor Adrian Fenty. "He started out the same way. He didn't listen to the people. Now he's an ex-mayor."
Nutter said he wants contracts that are fair to all city residents, not just union workers.
"These recent economic difficulties have just highlighted a structural problem with the city budget - too much money goes to pensions and health care and too little goes to city services, like libraries and recreation centers," Nutter responded by e-mail.
Staff writer Catherine Lucey and the Associated Press contributed to this report.