Pa. Registration Shifts Correspond With Vote Shifts
In the 2004, 2008, 2012, and 2016 presidential elections, the shift in counties' voter registrations correlated with shifts in how those counties voted for president.
This chart looks only at how numbers changed; for example, a county may have voted heavily Republican in two elections, but a slightly Democratic shift in registrations tended to then lead to a slight Democratic shift in votes.
Source:Inquirer analysis by JONATHAN LAI of data from Pennsylvania Department of State