Democratic Sen. Bob Casey is one of the longest-serving senators in Pennsylvania’s history. But his run of electoral success ended last month when he narrowly lost to Republican challenger Dave McCormick.

Now, with just weeks remaining in his 18-year Senate career, Casey is reflecting on what went wrong for him and his party in what was the most consequential battleground state on the map. Casey was the only Democratic Senate candidate running in a presidential battleground who lost his…