This kind of thinking assumes that the ruling party can unilaterally make policy changes, which is not possible in the American system currently (e.g. fillibust

This kind of thinking assumes that the ruling party can unilaterally make policy changes, which is not possible in the American system currently (e.g. fillibuster). w.r.t Sanders: he lost in a small d democratic primary. It wasn't the leadership: it was the voters that chose Biden over Sanders.