(3) The sheer size of the 2020 Democratic field will make personal attacks and exaggeration of issue differences unusually tempting. A candidate staring at a five-point deficit and an empty campaign treasury before a key, must-win primary would likely considering selling off their children for a well-timed day of media dominance, and unfortunately nothing works quite like a negative attack, whether it’s personal or ideological. But 2020 may be exactly the wrong year to assume Democrats can laugh off conflicts and kiss and make up after the primaries are over (if, indeed, the primaries even produce a clear winner). The only way to head off this dynamic is if other candidates along with party leaders and activists come down like the wrath of God on any candidate that succumbs to the temptation of straying over the line into attacks on a rival’s character or motives, or forgets to remind listeners that any differences on issues are laughably small when compared to the terrifying agenda of the GOP.

(4) It’s not enough for candidates to play nice with each other: they need to rebuke supporters who don’t and won’t. Anyone with the least understanding of social media knows that it won’t cut any ice if presidential candidates stay above-board while their most passionate supporters go after opponents with a tire iron – a tool that will be happily picked up by Team Trump the minute it’s discarded. Of course politicians can’t control everything their fans say and do. But public criticism may usefully shame the worst offenders into some self-control.