Bernie or Biden. Period.

If you’re a Democrat who doesn’t want Bernie Sanders to be the party’s nominee, your choice is now clear: You should vote for Joe Biden.

That’s the implication of Biden’s resounding victory in South Carolina. And Pete Buttigieg and Amy Klobuchar — to their credit — have both acknowledged this reality by dropping out of the race.

I realize it was only about a week ago that Biden’s campaign seemed grievously wounded, and he has still won only a single primary. So how has he so quickly become the only...