The state's April 22 primary is closed to voters not registered as Democrats or Republicans, and residents who want to vote in either party's primary have barely a month - until March 24 - to join or switch parties.
Both campaigns have begun setting up shop in Philadelphia and Pittsburgh in preparation what promises to be a protracted battle in Pennsylvania.
Obama's campaign plans "a very targeted and aggressive effort" to reach out to independents who support Obama, including Internet appeals, door-to-door canvassing, telephone calls and literature mailings, said Jeremy Bird, Pennsylvania field director for the Illinois senator's campaign.
From Clinton's campaign headquarters in Virginia, spokesman Blake Zeff said the New York senator's efforts include a special outreach to women independents.
"We consider independents to be extremely fertile ground for us in Pennsylvania," Zeff said.
Independents make up 12 percent of Pennsylvania's 8.1 million registered voters. There are 3.9 million Democrats and 3.2 million Republicans.
Obama is working to maintain the momentum that brought him 11 successive victories through Thursday, while Clinton is struggling to recapture her once-presumed front-runner status. The two are heading into a March 4 showdown that includes primaries in the delegate-heavy states of Texas and Ohio.
After that, if neither candidate can lay claim to the nomination, the Pennsylvania primary is the next high-stakes event in the delegate selection calendar leading to the national nominating convention in August in Denver.
As of Wednesday, Clinton needed to win 57 percent of the delegates in the remaining primaries and caucuses erase Obama's 89-delegate lead - a feat that would require her to win landslide victories, according to The Associated Press count.
At stake in the Pennsylvania primary are 158 of the state's 187 delegate slots.
Among the state's 29 superdelegates, 13 support Clinton, three prefer Obama and the rest are neutral.
The biggest concentrations of independents are in and around the Democratic hubs of Philadelphia and Pittsburgh.
Reaching outside the political parties to gain an electoral edge is nothing new in Pennsylvania.
When Gov. Ed Rendell was first elected in 2002, his campaign persuaded thousands of Republicans from Philadelphia's suburbs who fondly recalled his tenure as the city's mayor to temporarily switch parties so they could help him defeat now-Sen. Bob Casey in the Democratic primary.
David Sweet, who managed Rendell's campaign that year and is helping the Obama campaign this year, said the key to recruiting new party members from the ranks of the independents will be to muster enough volunteers to follow up and make sure the paperwork is filed on time.
"It's not easy to get people to switch," he said.
