Why did Howard Dean's scream destroy his campaign?

[Mr. Morden]

One lesson from the 2004 Iowa race is that when you have a large, competitive field of candidates on the Democratic side, the Iowa caucus polls might not be that great because of the caucus system there, which requires candidates to pass a 15% viability threshold at each precinct in order for those votes to be allocated.

Not such a big deal in 2016, because there were only two candidates of consequence, and not even that big a deal in 2008, because with three candidates splitting support almost evenly, they all still managed to reach viability in most precincts.  But with four or more candidates with double digit support statewide (like in 2004) there are going to be a ton of precincts where one or more of them has a respectable share of support, yet doesn't reach viability, which means that second choices become hugely important, and there's no good way to predict how things will turn out from the polling.  And so there's a very good chance that one or more candidates is going to do way better or way worse than the expectations set by the polls.

And since the main reason why Iowa and NH get disproportionate attention is because of "momentum", and momentum seems to be set relative to some kind of baseline of people's expectations going into election day, that makes Iowa (potentially) more important than usual when there's a large field of candidates with double digit support, because there's a greater chance that the polls will be "wrong".  I'd say that's something to keep in mind in 2020, since we may again have a large field of candidates who are competitive.