Jonathan Adler at Volokh:

I would think a mask requirement would be constitutional under Crawford v. Marion County Election Board, in which the Supreme Court upheld a photo ID requirement for voting in person. If the state’s interest in election integrity was sufficient to require a photo ID, the state’s interest in preventing the spread of Covid-19 should be sufficient to justify requiring a mask. The risk of Covid-19 is far greater than the risk of in-person voter fraud. The burden in Crawford was also deemed rather minimal, in large part because the state did not charge for identification. The burden was gong to get a qualifying ID, not paying for one.

Under Crawford, the easiest solution would be for polling locations to provide masks to those who show up to vote without one (much like many restaurants and stores do in jurisdictions with mask mandates), but this could be a significant added expense for election authorities, particularly in poorer jurisdictions. Election administration is underfunded in much of the country already, even without this added expense. Those communities that lack the resources to ensure an adequate number of voting machines are not likely to have the cash to spare for extra masks.

Easier absentee voting, early voting, and vote-by-mail would also lessen the burden of such a requirement, but not everyone can or will avail themselves of such opportunities. Presumably some share of those who wait to vote until election day are people who are less concerned about Covid-19, and who might be less inclined to wear a mask. We have all seen viral videos of people having meltdowns when asked to wear a mask to go into the supermarket. What happens when someone is denied the right to vote? If I am correct that Crawford controls (and I would be curious whether readers disagree with me on that point), I think election authorities could deny entrance to a would-be voter without a mask, particularly if masks were made available to those who come without one.