A head to head comparison of the 2008 Pennsylvania General Election gives us an idea of the break down of the voters.    

Barack Obama received 3,266,523 (55%) votes, beating John McCain who gained 2,649,934 (44%) by 616,589 votes in Pennsylvania.

John McCain won 49 counties whereas Barack Obama won 18 counties in the total of the 67 counties in the State of Pennsylvania.

Presidential General Elections

During the 2008 General elections, over 131M total number of votes were cast. Barack Obama got 69,448,278 (69.4 Million) over John McCain's 61,739,059 (61.7 Million). He won the popular vote by 7,709,219 (7.7 Million). His victory in the State of Pennsylvania (with 3,266,523 votes) represents 4.5% of his popular vote victory. The remaining 49 states contributed to the remaining 95.5% popular votes victory.

Top 15 Counties in Primary Elections vs General Elections

All Candidates (General Elections)

These 15 counties play an important role in these Primary elections. If a candidate, can get a sizable majority in the largest counties (Philadelphia, Allegheny, Montgomery, Bucks), it becomes very difficult to win majorities in the smaller counties without voter outreach and increased voter participation in them.

Top 15 Counties Comparison in Primary Elections vs General Elections

General Elections
Barack Obama

General Elections
John McCain

Philadelphia, Allegheny, Montgomery and Bucks, four of the larger counties voted in favor of Barack Obama, giving him a 696K vote advantage. However, steady marginal wins for John McCain in the less popular counties (like Lancaster, Westmoreland and more) were not enough to overcome that deficit, making Barack Obama gain an upper hand in the State of Pennsylvania.

Related References: 2016 Pennsylvania Primary Election Story, 2016 Pennsylvania General Election Story, 2020 Pennsylvania Primary Election Story, 2020 Pennsylvania General Election Story, 2012 Pennsylvania General Election Story