The cleanest summary of the Pennsylvania situation is from the MSNBC’s First Read guys (Chuck Todd, Mark Murray, Domenico Montanaro) who always deliver neat, comprehensive and super-astute summaries of everything that’s going on in the political world.

“There seem to be four possible outcomes to tonight√ɬ¢√¢‚Äö¬¨√¢‚Äû¬¢s contest, and two of them will need little spin because the media won’t need the ‘help’ to interpret their meaning: 1) a double-digit Clinton victory in which she beats Obama by a greater margin than she beat him in Ohio; and 2) an outright Obama victory. But here are outcomes that will force the campaigns to go into spin overdrive: 3) a Clinton victory by less than five points, which would give Obama an opening to declare ‘victory’ of sorts and create renewed pressure on Clinton on the future of her campaign; and 4) a Clinton victory by more than five but less than 10, which is the most likely result if some of the better polls are to be believed.
“[The last scenario] would be considered a solid victory, but would it be big enough to fundamentally change the dynamics of the race? Could Clinton claim Obama was losing ground if he performed better in Pennsylvania than he did in Ohio? This is why Axelrod and his team and Wolfson and his team will earn their money today. The Clinton campaign is desperately trying to ratchet back expectations.
“Early on, Pennsylvania Gov. Ed Rendell and his folks were predicting an easy victory. Those predictions, however, are what created this expectations-game problem for Clinton today. She’s now in a position where only a blowout is going to give her the momentum she needs to make folks believe she still has a shot at the nomination.”