WASHINGTON — President Barack Obama headed to Pennsylvania today to raise money for Democratic Senate candidate Joe Sestak, who is locked in a tight race for a seat considered a must-win for the president's party.

Democrats know Sestak's chances of winning depend heavily on the party's ability to ramp up voter turnout in cities like Philadelphia. With six weeks until the midterm elections, Obama is trying to fire up the party's base, urging the first-time voters who helped him win the White House in 2008 to head back to the polls in November.

"I need everybody here to go back to your neighborhoods, and your workplaces, to your churches, and barbershops, and beauty shops," Obama told a Congressional Black Caucus dinner Saturday night. "Tell them we have more work to do. Tell them we can't wait to organize. Tell them that the time for action is now."

The relationship between the White House and Sestak has been a rocky one. Obama backed longtime Republican-turned-Democratic Sen. Arlen Specter in the primary contest Sestak won earlier this year. The White House had tried to get Sestak to drop his challenge to Specter by offering him an unpaid presidential advisory position, an offer Sestak rejected.

Obama carried Pennsylvania during his White House run, but two years later, voters are angry at the sluggish economic recovery and could take their frustrations out on Democrats in November's balloting. Democrats see the Pennsylvania Senate seat as one that could not only determine which party holds the Senate majority, but also which way the key swing state might lean in 2012.

Several polls suggest Republican Pat Toomey, a former congressman and ex-head of the conservative Club for Growth, has a lead over Sestak. Republicans need to capture 10 Senate seats to win a majority. As many as a dozen held by Democrats appear competitive — the Pennsylvania race included — as well as at least five currently in the hands of the GOP.

Obama's trip to Pennsylvania is the latest in a string of appearances by Democratic heavyweights.