A lot of people called for Graham Platner to drop out of Maine’s Senate race Monday night, after a rape allegation surfaced against the Democratic nominee. But relatively few of them were Republicans. Even Platner’s GOP opponent, Sen. Susan Collins, called the allegations against the oysterman “appalling” but added that it was “not up to me to choose the Democratic nominee for Senate.” There’s a simple reason for that: Democrats’ hopes of winning the seat and, by extension, the Senate, could hinge on Platner stepping aside. For now, Platner is still in the race, and he denies the allegation from...