In the end, a direct measure of partisanship is the strongest predictor of vote choice, despite what you’ve perhaps read about fancy microtargeting models that claim to nail down attitudes of voters based on something like the car they drive. The simple, old-fashioned measures of partisanship do the bulk of the work in the models and the polls.

But Wisconsin is one of a handful of states where none of this data is available. There’s no party registration, and the voter file indicates only whether someone voted in a primary, not which one. Minnesota, North Dakota and Montana are also in this category.

It’s an even greater challenge because these states, with a lot of white, rural residents, have relatively few other characteristics that strongly correlate with partisan vote choice.

In other parts of the country, weighting — giving more weight to respondents from underrepresented groups, to ensure the sample reflects the demographic profile of likely voters — can get you a long way because characteristics like race, age and education are also correlated with partisan vote choice.

In our i nitial Wisconsin and Minnesota polls, we took an all-of-the above approach. Not only did we weight by those standard categories, but we also tried to estimate the likely partisanship of our respondents using our prior polling data. Ultimately, this was only somewhat successful.

So far, we’re observing a pretty modest response bias toward Democrats, particularly in well-educated districts. You can see that for yourself by looking at the “if we didn’t weight by party” option in the weighting section of our live poll pages. This would seem to imply that, in general, we’d be at a greater risk of overestimating Democrats in places like Minnesota or Wisconsin.

We’re not going to shy away from polling the most important districts. But at the margins, we’d prefer to poll in the places where we can leverage as much data as we can. Minnesota’s First District, for instance, is the kind of race that we would probably poll if it had party registration. We’ll probably avoid it.