I made a scatterplot comparing the change in coronavirus cases over the last two weeks to cooling degree days—a proxy for air conditioning use. (Thanks @jeremy_gibbs!) The hottest states have suffered the worst outbreaks. 
I think this effect explains a lot of the partisan divergence in this chart. The hottest states are all Republican. Still, at any given temperature Democratic states seem to doing modestly better, on average, than Republican states.
Lest you think the recent increase in southern states is an artifact of testing: a scatterplot of the positive coronavirus test rate shows a similar relationship. The hottest states are seeing coronavirus infections outstrip testing capacity.
Made a quick and dirty scatterplot comparing late March covid increase to March heating degree days. No obvious relationship.
