But in Milwaukee, where there are more than 1,000 confirmed cases of the virus and at least 87 people have died of it, many voters cast their ballots wearing full protective gear, some overtaken with fear.

Some Republican efforts to downplay the danger of the election ended up highlighting the medical risks involved. For instance, Robin Vos, the Republican speaker of the Wisconsin Assembly, posted on Facebook that he was volunteering as a poll worker, writing that “an impressive amount of planning and organization” went into securing the election. In photos, Mr. Vos looked ready to enter a contaminated zone: He was wearing a face mask, a plastic body covering and gloves.

The partisan divide within Wisconsin over the safety and integrity of the election was mirrored on the national level, with the Democratic presidential candidates raising concerns about the safety of the vote and President Trump urging Republican voters to the polls as though little was out of the ordinary. On Twitter, Mr. Trump lauded the incumbent Wisconsin Supreme Court justice seeking re-election, Daniel Kelly, as a jurist who “loves your Military, Vets, Farmers.” And later in the day he weighed in on the vote by mail effort, saying without evidence, “The mail ballots are corrupt, in my opinion.’'

“Mail ballots, they cheat,’’ the president said at his evening briefing on the virus. “Mail ballots are very dangerous for this country because of cheaters. They go collect them. They are fraudulent in many cases.’’

Mr. Trump himself voted by mail in the 2018 midterms and in last month’s Florida primary.

Mr. Sanders, who like Mr. Biden has held no campaign events in nearly a month because of the virus, took a far more somber approach, rebuking Wisconsin Republicans for risking “the health and safety of many thousands of Wisconsin voters” to force an election under conditions of extreme adversity.