One of the main takeaways from 2016 was that many state-level polls underrepresented the number of white voters without a four-year college degree in the electorate, a group that overwhelmingly backed Trump in 2016. As a result, some pollsters have since weighted their surveys by education, but not all have. It remains a critical cleavage that pollsters still need to address.

Weighting for education is complicated, though; namely, highly educated Americans are more likely to answer a poll than less educated Americans, and in an era of increasingly lower response rates, this could be a problem as underlying samples may be more educated than the actual population. The upshot is that even if a pollster is weighting its respondents to match the educational breakdown of a state’s population, it still may not be reaching enough voters with lower education levels. This could in turn affect the poll’s findings for the presidential race because the educational divide is now one of the most important predictors of party support besides race.

This means that pollsters could once again find themselves underestimating Republican support in 2020 because of weighting and sampling issues. After all, separate analyses by Sean Trende at RealClearPolitics and Nate Cohn at the New York Times’ The Upshot both found that after the polls undershot Trump’s support in 2016 in key battleground states like Florida, Iowa, Michigan and Ohio, the same thing happened in 2018 with those states underestimating Republican Senate and gubernatorial candidates. Overall, though, the polls were pretty good in 2018.