For a fractious coalition that at times seems to be held together with spit, baling wire, and old memories of Barack Obama, the Democratic Party has a remarkably singular message: Donald Trump is an existential threat to the country. For the Biden campaign, the existential threat to democracy has become the overriding theme in his bid for reelection. “There is one existential threat: It’s Donald Trump,” Biden said at a fundraiser in February. For the environmental activists in...