AP Photo/Charles Rex ArbogastSupporters listen to Democratic presidential hopeful, Sen. Barack Obama, D-Ill., at a rally in Scranton, Pa.
PHILADELPHIA " Older, whiter and more female than the nation as a whole, Pennsylvania looks like Hillary Rodham Clinton country.
Wealthier, better educated and more African American than the rest of the state, Pennsylvania's thickly settled southeast corner could belong to Barack Obama.
For six weeks, the two Democratic presidential rivals have courted their political bases and sought to carve up each other's support with an increasingly tart tone. After both deluged the state with ads, crisscrossed it on buses, planes and trains, and had a parade of surrogates march through it, Clinton holds a slight lead.
Pennsylvania's primary Tuesday could be decisive, or it could extend a campaign that has lasted longer than most ever imagined.
Clinton, the New York senator, is looking to validate her case that only she can win big-state primaries. Obama, the Illinois senator who leads in delegates and in the national popular vote, wants to shut down Clinton so he can turn his attention to John McCain, the likely Republican nominee.
A defeat for Clinton could be devastating. One of her key backers, New Jersey Gov. Jon Corzine, said last week that a Pennsylvania loss would be "pretty much a door closer."
At stake are 158 delegates, with 103 apportioned according to how each candidate fares in each of Pennsylvania's 19 congressional districts. The remainder are distributed based on the statewide vote.
The state has 29 superdelegates " party officials and elected officials who can support whomever they chose. Fifteen have endorsed Clinton and five have endorsed Obama.
Hillary Clinton, as the state's old slogan used to say, has a friend in Pennsylvania. Its population of 12.4 million has a higher median age, a higher percentage of whites, a lower median household income and fewer bachelor's degrees than the country overall.
These are the voters " working-class whites and voters older than 50 " who have flocked to her in past contests.
Obama and his allies are counting on Philadelphia and its suburbs to be the underpinnings of his campaign. They also hope to do well in Pittsburgh.
Philadelphia is 44 percent black and Pittsburgh is 26 percent black. Both are higher education hubs, with students and academics who tend to be Obama voters.
Clinton has drawn more conservative voters, whereas the two cities have Democrats who are more liberal than in the rest of the state.
Moreover, Philadelphia's surrounding counties also trend in favor of Obama with their greater number of upscale voters and college students.
"Southeast Pennsylvania will go for Obama," said Robert Maranto, a political scientist at Villanova University. "There's a huge black vote in Philadelphia. The suburbs are a lot more pro-Obama.
"The rest of the state should go pretty heavily for Clinton," he added.
Obama's support in Philadelphia was evident by a rally there Friday that drew 35,000 people, the largest crowd of his campaign.
Yet Clinton has the backing of Gov. Ed Rendell, the city's popular former mayor, and current Philadelphia Mayor Michael Nutter, one of the state's prominent black leaders.
Obama, on the other hand, was endorsed by Sen. Bob Casey, a member of a prominent political family from the state's more conservative northeast.
Casey has spent considerable time at Obama's side, traveling with him on tours through the state.
Another statistic that could influence the outcome: Obama has outspent Clinton by as much as 3-to-1 in the state.
The Clinton campaign estimates he has spent about $11 million in ads to her nearly $5 million " a ratio confirmed by independent analysts.
Pennsylvania is the sixth largest state and the 41st to vote in this Democratic primary.
It has about 4 million registered Democrats, thousands of them added in a recent registration surge. None of the remaining nine states is as big or as rich in delegates.
Between the two major cities are 7.7 million acres of farmland, with the kinds of small towns that harbor more conservative voters who, if not Republican, would be more likely to vote for Clinton.
The state's steel, coal and railroad industries have atrophied, contributing to the slowest population growth of any major state.
Fifteen percent of the state's population is over age 65; only Florida and West Virginia have a higher ratio of seniors. Conversely, 10 percent of state residents are between age 18 and 24, a reliable pro-Obama cohort.
Eighty-two percent of the state's population is white, compared with 66 percent for the nation. And 46 percent of the state's voters are gun owners; Clinton has been assiduously courting the subset of that group that votes Democratic.
White men have been a key swing group this year. Clinton won them in Ohio, tied with Obama for their votes in Texas and came close in Rhode Island. In that regard, certain areas of Pennsylvania are key.
Chief among them is the Lehigh Valley, a once down-on-its-luck region that has made a comeback and seen a population increase. This eastern corridor consists of the cities of Allentown, Bethlehem and Easton. It has more whites than the state as a whole and is considered home to the original "Reagan Democrats." The valley also is a higher education center, with schools such as Lehigh University and Lafayette College, and has a burgeoning health care industry.
While Clinton is looking for a victory, Obama and his allies are willing to settle for a close contest and a shot at getting more delegates than she does.
The delegates are distributed to the 19 congressional districts based on how Democrats performed in 2004 and in the 2006 governor's race. Congressional districts with larger numbers of delegates at stake tend to favor Obama.
Seven of the state's 19 congressional districts yield 50 of the state's 103 elected delegates. All but one of those districts are concentrated in and around Philadelphia.