We saw what we saw. Should the Democratic Party try to replace Joe Biden as its nominee—and if so, with whom? The stakes were sky-high last week, and this week, the Supreme Court made them virtually existential: It granted Donald Trump immunity for any official acts he might take if returned to the White House. Literally, any alternative would be better than that: America has had presidents with severe physical and mental impairments, abominable judgment, or the morals of an alley-cat—but we’ve...