This isn’t the cakewalk she expected. While not mentioning his name often, Hillary Clinton has tried marginalizing Bernie Sanders by moving left, narrowing the distance between them on income inequality and Wall Street regulation, then whacking him occasionally for opposing increased gun control. It hasn’t worked.

Last month Mrs. Clinton won all eight of the polls taken in Iowa, beating Bernie by an average of 16 points. But now her edge has fallen to only four points in the Real Clear Politics average for the state. Even...