Barack Obama is speaking at four separate rallies around Philadelphia today -- for good reason. Support in and around Philly is central to a Democrat's chances of winning the state.

In 2004, John Kerry carried Pennsylvania by about 145,000 votes -- just a 2.5-point margin. But in Philadelphia County alone, Kerry pummeled Pres. Bush, racking up more than 400,000 more votes than the president, thus allowing him to lose other less-populated portions of the state.

Democrat Bob Casey took 290,000 more votes in Philadelphia County than Republican Sen. Rick Santorum in their 2006 Senate race. Casey would cruise to victory statewide, helped in large part by his 84% take in the county.

In the April Democratic primary, Obama took close to 300,000 votes in the county -- just 70,000 less votes than Casey won in the 2006 general election. Combined with Hillary Clinton, the two Democrats took in a total of some 442,000 votes, only 100,000 less than Kerry won in the county in the 2004 general.

Obama appears set for a result in the state that will more closely resemble Casey's win than Kerry's. Obama's lead has continued to grow since mid-September, when the RCP polling average margin between Obama and McCain was within just a couple of points. Obama now leads by 13.8 points in the RCP Average for Pennsylvania.