 Solution

First, compare counties only after matching, weighting, or regression adjustment for baseline prevalence, calendar date, population density, age structure, household size, income, testing intensity, and public-health restrictions. Use county-clustered uncertainty to respect within-county dependence.

Second, replace the coarse red/blue split by continuous vote share and estimate a prespecified birthday-event-by-vote-share interaction. A continuous analysis retains information, permits a dose-response check, and avoids sensitivity to an arbitrary 50% cutoff. Reporting subgroup sample sizes and correcting for multiple subgroup searches would further reduce selective interpretation.