The Pennsylvania preview in today's Times makes a great point:  

In an atmosphere where both sides are hedging their expectations, Clinton aides have refused to say what margin of the popular vote she needs to win to stay in the race. The contest for delegates, who are awarded proportionally based on how well the candidates perform in each Congressional district, is likely to be close, but the pressure is on Mrs. Clinton to get at least 50 percent of the delegates.

“The fact that Hillary is crisscrossing a lot of Congressional districts, and Bill is, too, is proof that while everyone is focused on the vote percentages statewide, there is a war for delegates,” said Tony Podesta, who has run several statewide races in Pennsylvania and supports Mrs. Clinton, referring to former President Bill Clinton. “She needs to find ways of closing the delegate gap; she can’t go through all these contests and split the delegates 50-50.”...

For his part, Mr. Obama has devoted his time to those same suburbs and reached beyond them to the exurbs, trying to appeal to well-educated, liberal, affluent voters for whom the war in Iraq is a central issue. While his most reliable base is made up of black voters, he has steered clear of Mr. Fattah’s district, the heavily black area of Philadelphia in which Mr. Obama expects to win the most votes and the most delegates. Instead, he has campaigned in each corner of the state, making forays into Mrs. Clinton’s base and trying to capture some of those delegates.

So far as I can tell, Obama is spending most of the day in the western part of the state. At first glance, you might wonder why he'd waste his time there, given that he's likely to lose many of those counties by at least 20 points. But, as the article suggests, that's exactly the point. Hillary would have to win the local congressional districts by better than a 60-40 margin (I think) to really pile up delegates. If Obama can hold her a liittle closer, he has a chance of making the delegate haul from Pennsylvania into something close to a draw.

--Noam Scheiber