... People usually don't know why they vote for the candidates they choose to vote for, and are not particularly good at assessing how something influenced that vote -- let alone how some hypothetical future event would influence them.

...if you ask voters, it turns out that some will tell you that they would be more likely, and a somewhat larger number will tell you that they'll be less likely, to vote for someone with a Trump endorsement. Hey, reporters: don't believe those polls! You can take it as a measure of what respondents think about Trump, if you care about such things, but there's no reason to believe that this kind of self-reporting about vote choice is meaningful at all, and it shouldn't be included in stories about a Trump endorsement as if it was meaningful.

...The bottom line here is that polling is a really good tool for reporters to use in many cases, but remember: what polling tells you for sure is only what people will say if they're asked a question by a pollster.

Jonathan Bernstein