Elections are a form of communication. Voters send messages to politicians and the press about what they will tolerate and what they won’t.

The Democrats haven’t voted yet, but they’ve been campaigning for more than a year. They have just held their last debate before the Iowa caucuses two weeks from Monday. That’s time enough to learn some useful things, from the majority of the two-dozen-plus declared candidates who have already dropped out and from those still in the race.