From a post on the Pennsylvania primary election:
As this CQ Politics article points out, regions in Pennsylvania that leaned toward Democrats in the most recent presidential and gubernatorial elections received more delegates from the state party. In addition, the state’s 19 congressional districts use a proportional system to determine how many delegates a candidate receives at the convention. Hillary Clinton and Barack Obama will easily clear the 15% threshold mandated by the DNC, but the number of delegates they receive will depend on how well they perform in the districts.
As this chart shows, candidates need a very high percentage of the vote to really “win” a district. For example, unless a candidate breaks the 58.3% mark in CD-6, Clinton and Obama will split the six delegates there. And while anything over 50% in CD-14 will give a candidate a 4-3 delegate advantage, they would need to reach 64.3% to net a 5-2 win. It’s for that reason that CQ Politcs predicted a slim 53-50 delegate spread for Clinton, despite her relatively sizable and constant polling lead in the state.
We’ll keep Pennsylvania’s congressional and presidential pages updated as best we can throughout the night and tomorrow. If you’re looking for the most complete picture around (from district-by-district vote results to updates on the superdelegates) Congresspedia is the place to be.