At what point does a politician’s behavior become so scandalous or embarrassing that he, or much more rarely she, has no choice but to resign or drop out of a campaign? This used to be a relatively straightforward question—something as simple as being caught having an extramarital affair could be disqualifying. But things are different in the Donald Trump-era Republican Party. Trump, as all Americans recall very fondly, chose not to drop out of the 2016 presidential race after the release of an audio tape in which he described his enthusiasm for touching women in a way that really really...