In fact, Ms. Selzer doesn’t weight her polls nearly as much as other R.D.D. pollsters, including the ABC/Post poll. Rather than weight on a bunch of different variables, like population density, education, race and so on, she uses a technique called post-stratification weighting. That’s a fancy way of saying she splits her sample into tiny groups of voters — by age, congressional district and gender (say young men in Iowa’s First district) — and makes sure that each group represents the right share of the sample.

And this year’s Selzer poll looks a lot more like an outlier than the Selzer poll did four years ago. Back then, the Monmouth poll in September showed Mr. Trump up by eight points; this cycle, Monmouth’s poll in October showed Mr. Biden ahead. More broadly, national polling four years ago showed Mr. Trump running very well among white voters without a degree, who are overrepresented in Iowa. This year, polls show Mr. Biden doing far better among white voters without a college degree.

The Biden campaign certainly seems to think it’s doing well. Mr. Biden visited Iowa on Friday, and there’s not really any chance he’d be visiting there if the campaign didn’t think he was tied or ahead. Of course, campaigns have been wrong before. But so has Ms. Selzer.