Shifting vote margins in the hours and days following the close of polls was a highlighted issue in the 2020 election. The tendency of vote margins in recent presidential elections to trend in favor of the Democratic presidential candidate as the count proceeds has been previously studied and given the label “blue shift.” We address how these shifts occurred during the 2020 election with contemporaneous National Election Pool (NEP) data reported by county. States were slower to report overall vote counts if they had large volumes of mail ballots, prohibited preprocessing mail ballots, and allowed mail ballots to arrive after Election Day. We also find that both between- and within-county differences drove partisan trends in the 2020 vote count. Counties Biden won took longer to complete their counts than counties Trump won. Within the average county, Biden’s votes took longer to count completely than Trump’s. Nonetheless, in the first couple of hours after polls closed, there was actually a disproportionate number of Biden votes reported, which we show were likely due to preprocessed mail ballots being reported first.