With just two much smaller contests between now and the state's April 22 primary - in Wyoming and Mississippi - Pennsylvania is in for a marathon of rallies, town-hall meetings, television ads and high stakes get-out-the-vote efforts.
"We're going to get every bit, if not more, than the voters in Iowa and New Hampshire got," boasted Philadelphia lawyer Mark Aronchick, a national fundraiser for Clinton's campaign. Mark Alderman, a national fundraiser for Obama, agreed that Pennsylvania is going to "look more like the Iowa campaign than anything since Iowa."
One additional wrinkle in Pennsylvania: Only Democrats can vote in the Democratic primary; independents, who have strongly supported Obama in other states, are barred. But the campaigns have until March 24 to persuade the state's 984,000 registered voters who are not members of either major party - plus any wavering Republicans - to sign up as Democrats so they can vote in the primary.
The nation's sixth most populous state, Pennsylvania bears many similarities to Ohio, where Clinton defeated Obama handily.
Two major metropolises - Philadelphia in the southeast and Pittsburgh in the southwest - bookend a vast rural region with 58,000 farms on 7.7 million acres - an area larger than Massachusetts.
It's a Rust Belt state largely abandoned by the once-mighty steel, coal and railroad industries. Today, its biggest employers are the federal government, the state government and Wal-Mart, in that order.
Pennsylvania's comparatively high union membership - 13.5 percent of state wage earners compared with 12 percent nationally - and large elderly population - only Florida and West Virginia exceed its 15 percent aged 65 or older - make it fertile ground for Clinton, whose political base is anchored by older white voters and blue-collar workers. The state AFL-CIO estimates a third of the registered voters live in union households.
Only 10 percent of Pennsylvanians are between age 18 and 24, a group that Obama has captured in other states.
Political observers expect Obama will do well in Philadelphia, the state's Democratic hub, where more than 40 percent of the residents are black, and among the younger, better educated voters in the city's suburbs. Clinton, they say, may do better among more conservative, working class Democrats in northeastern and southwestern Pennsylvania.
The state has a slim track record of electing women and blacks to public office - relevant in a year when Democrats likely will have either the first female or first black nominee for president. Women comprise just 13 percent of the state legislature, in contrast to the national average of 23 percent. Blacks held 8 percent of the seats, equal to the national average.
In Washington, Pennsylvania's 21-member congressional delegation includes one woman and one black.
Of the state's 29 superdelegates - the officeholders and party leaders not bound by the primary vote - 13 have endorsed Clinton while four back Obama.
Most polls show Clinton leading in the state, but the margin has shrunk in recent weeks. A Quinnipiac University poll released last week showed Clinton with 49 percent of the vote and Obama with 43 percent.
Clinton has the backing of Gov. Ed Rendell and Philadelphia Mayor Michael Nutter, one of the state's prominent black leaders.
The last time Pennsylvania's primary made a difference was in 1976, when former Georgia Gov. Jimmy Carter's victory cleared the way for him to win the nomination and, in turn, the presidency.
Registered Democrats numbered 3.9 million last fall, but officials in many counties say their ranks have swelled with voter interest in the Obama-Clinton contest. The 8.1 million voters registered for last year's elections included 3.2 million Republicans as well as the 984,000 voters not registered with either major party.
In presidential elections, Pennsylvania has been a "swing state leaning Democratic," according to Terry Madonna, a pollster at Franklin & Marshall College.
Democrats Al Gore and John Kerry carried the state in 2000 and 2004 while losing nationally to Republican George W. Bush. In the previous 12 presidential elections, Pennsylvania voted for the winner 11 times, siding with only one loser, Democrat Hubert H. Humphrey in 1968.
