Of all the mini-battlegrounds within Wisconsin — perhaps the most pivotal state in November for both President Trump and Joseph R. Biden Jr. — the mother lode of absentee ballots is coming in Dane County, a Democratic stronghold that includes Madison. As of Friday, the number of submitted ballots there amounted to more than 36 percent of the county’s total 2016 election vote, a sign of significant enthusiasm; that figure is 10 percentage points higher than in any other county in the state.

In Wisconsin’s Republican heartland, the suburban counties that ring Milwaukee, the absentee turnout is only at about the state average so far. And in the dozens of rural counties where President Trump won huge victories four years ago, ballots are being returned at a far slower rate than in the state’s Democratic areas.

The yawning disparities in voting across Wisconsin and several other key battlegrounds so far are among the clearest signs yet this fall that the Democratic embrace of absentee voting is resulting in head starts for the party ahead of Election Day. For Republicans, the voting patterns underscore the huge bet they are placing on high turnout on Nov. 3, even as states like Wisconsin face safety concerns at polling sites given the spikes in coronavirus cases.