Pennsylvania: Watch Chester County

Pennsylvania Gov. Ed Rendell (D), during an appearance moments ago on MSNBC, predicted that Barack Obama would emerge with a 650,000-vote edge out of Philadelphia and the surrounding suburbs -- an analysis that has set off speculation that John McCain will trail too badly in southeastern Pennsylvania to make up the margin elsewhere.

Pennsylvania's polls don't close until 8 p.m. ET so let's not get ahead of ourselves.

When the polls do close, make sure to keep an eye on Chester County located just south and west of the city of Philadelphia.

Chester is the only one of the four suburban Philly counties that went for George W. Bush in 2004 and Republicans have cast it as a McCain must-win for his hopes statewide. Chester has been moving to Democrats over the last few years, however, as Sen. Bob Casey Jr. (D) and Rendell himself both carried the county in 2006.

Watch Chester. If Rendell is right about Obama's margins in southeastern Pennsylvania, the Illinois senator will be sitting pretty in the Keystone State.