Laurence H. Tribe, a professor of constitutional law at Harvard, is the co-author of “To End a Presidency: The Power of Impeachment.” Imagine if in 1972, President Richard M. Nixon, rather than routing his opponent, had barely won the presidential election. In that world, when evidence came to light that Nixon had committed impeachable offenses during his campaign, it would have been plausible to suppose that those offenses were essential to his victory. And the stakes would then have been even higher than they were when, in 1974, the Supreme Court was deciding whether Nixon had to comply with a...