A voter’s party registration is a strong indicator of who they’ll support, but it’s not a guarantee. In fact, many voters registered with one party have actually been voting for the other party in recent elections but haven’t necessarily switched their registration to reflect the party they actually support.

Take Pennsylvania, for example. The once-Democratic southwestern part has shifted sharply toward the GOP over the past couple of decades. However, party registration figures haven’t necessarily reflected that movement as much as you might expect. For instance, Greene County along the West Virginia border voted for Trump by 40 percentage points in 2016, yet preelection registration figures1 show that party identification is split almost evenly, with registered Republicans and Democrats each making up 45 percent of the county’s voters.

Part of what’s going on is that many older voters in that region are still registered as Democrats, even if they back Republicans for most federal offices. Conversely, the suburban counties around Philadelphia in the eastern part of the state used to form the base of the state Republican Party, but even though that area has moved toward the Democrats in recent elections, some Democratic-leaning voters haven’t changed their party registration. In other words, big shifts in party registration sometimes tell us something we already know, and aren’t a signal of a new shift in attitudes.