The Democratic Party has spent the past year winning elections without settling a basic question: what it wants to be. Its voters keep giving split answers. In some places, they choose Democratic Socialists who vow to upend the system; in others, moderates who promise to steady it. Both keep winning. And because the two tend to win in different kinds of districts—the left in safe, deep-blue seats, moderates in the purple and red places that decide majorities—the party has not had to choose between them. The factions rarely share a ballot, so the disagreement can stay theoretical. That, strategists and elected officials across the party’s wings suggest, is much of why a reckoning over its direction has not yet arrived. It is coming. For now, geography is holding it off....