Donald Trump is facing an extreme sliding-doors scenario on Election Day. If he wins, he would have the power to single-handedly scuttle the federal criminal cases against him, be immune from prosecution while in office, and, thanks to the Supreme Court, have broad immunity from prosecution once — if — he leaves. If he loses, though, he will face criminal penalties that could leave him in command of a 70-square-foot prison cell for most of the rest of his life. “He will be facing serious legal jeopardy if he loses. He knows that,” says Bennett Gershman, a professor of constitutional...