The Keystone State Primary: It's a regional contest with two teams that represent two different Pennsylvanias.
"What we have in Pennsylvania at the moment is a sort of East /West contest," said G. Terry Madonna, Director of the Center for Politics and Public Affairs at Franklin and Marshall College. "We joke about it in Pennsylvania. The Philadelphia Eagles playing the Pittsburgh Steelers."
Pittsburg Steelers fans are typically Clinton voters. Many are Catholics and blue-collar workers from central and western Pennsylvania. They want someone to fight for health care, but are more culturally conservative.
Philadelphia Eagles fans tend to fly with Obama. They are more educated, affluent, and liberal voters who live in the Eastern part of the state, including Philadelphia, which is evenly split between black and white voters.
Many compare this battleground with Ohio, its sister swing state to the west, but because of some key differences, they're really more like cousins than siblings.
"There are more Catholics in Pennsylvania than Ohio. More union members in Pennsylvania than Ohio. More older voters in Pennsylvania than Ohio," Madonna said.
And more women … all factors that could move Clinton close to a big win.
But Obama's greater resources could keep the game close.
"I think his advertising budget has been three times that of Sen. Clinton which obviously has helped him enormously," said Barbara Ferman, a political science professor.
There also may be power in numbers. Many speculate a surge of new Democrats will help Obama.
"Since the campaign started, there has been slightly more than 300,000 Democrats registered in Pennsylvania.
The Keystone State is key in the general election, too. Consistently courted by the GOP, Pennsylvania is a state McCain hopes to change from blue to red. And it's not hard to understand why.
"Of the 10 closest states in 2004, for example Pennsylvania had the largest number of electoral votes," Madonna said.