Great post from Dan Hopkins about how a belief that "messaging" works gets politicians into trouble.
What I've become interested in are the incentives for politicians to believe the myth. There are a couple of things going on here, I think.
One is to recognize the distinction between candidates and candidacies and the analogous distinction between elected officials and their full operation, their offices (we don't really even have a proper word for that one). Within those candidacies/offices, there are a number of people -- the communications operation, probably polling, media, maybe more) who have a direct interest in believing that the words that politicians say have a direct effect on public opinion in a way that really matters.
The other one is about efficacy.
In some situations -- campaigns, in particular -- "messaging" actually does have some marginal utility, and while it's only on the margins, it's one thing that is actually within the control of the candidates, so they might as well do it as well as possible.
But even when it has no effect whatsoever on public opinion, it's likely to produce tangible results. Take for example the House mini-CRs during the shutdown, or the First and Second Battles of the World War II Memorial. We know that they were utterly unsuccessful at moving public opinion. However, they also were heavily reported, especially within the GOP-aligned press. In other words, they had tangible effects, even if they didn't actually have meaningful effects. And within most organizations (and remember, House offices, Senate offices, political parties, the White House, campaigns, and federal agencies are all organizations), tangible accomplishments tend to be rewarded. Even if tangible may not, at the end of the day, be as substantive as something else which doesn't have any visible gain that one can point to.
And as I've said, I think this is particularly a problem for Republicans, both because their party-aligned press is a bigger deal, and because it is generally even more accepting of GOP spin than the Democratic-aligned press is. Which means that GOP spin becomes tangible more easily, which should mean, all else equal, that Republicans will be even more susceptible to the trouble that Hopkins talks about than Democrats will be.
Also: Nice catch!