Governor’s races in South and Midwest could be lost if party brand becomes too liberal. Centrist Democrats were wiped out in the 2014 elections and in their absence emerged a resurgent liberal movement, embodied most recently by the surprisingly competitive presidential campaign of Vermont Sen. Bernie Sanders. But the suddenly ascendant left — its populist overtones becoming part of the mainstream Democratic pitch — is worrying Democrats who want to compete on Republican-leaning turf. The party lost every competitive gubernatorial and Senate race in the South last year. And Democrats didn’t fare much better in the heartland. Now, as Bernie...