Targeted count in two counties heavily won by Biden. Total votes cast represent one-quarter of the total votes cast in WI.
Dane County--345K votes cast. Biden won 75.5% of vote.
Milwaukee County -- 459k votes cast. Biden won 69% of vote.
Those two counties account for 35% of Biden vote in WI.
Looking at Milwaukee County, Biden received 69% of the recorded vote, vs. 65% for Hillary. Trump's percentage remained flat at 29% in both years, with 6K more votes for the President from 2016 to 2020. Biden received 36K more votes than Hillary. Third party vote declined from 7% to 1.6% between 2016 and 2020, about 21K fewer votes. About half of that vote went to the Libertarian candidate. These voters, if they defected, would more likely favor Trump vs. Biden. About one-fourth went to the Green Party, whose voters would support Biden. The main difference between 2016 and 2020 is the 24K vote increase in overall vote count. Staring with Hillary's 281K base, and adding about 5K Green Party switches to Biden, that would mean that all the 24K vote increase between 2016 and 2020 would have to go to Biden, which is impossible.
Admittedly, this is not proof, but this is just one of many improbabilities.