For a number of states that already allow a large share of voters to vote remotely, the road would be considerably smoother. Five states — Colorado, Hawaii, Oregon, Utah and Washington — conduct elections almost entirely by mail. Twenty-eight others and the District of Columbia allow voters to cast absentee ballots by mail without providing a reason, though participation is dampened in states that make voters apply for ballots in every election instead of providing them automatically.

Some of those states have impressive rates of balloting by mail, including big states like Arizona, California and Florida and smaller ones like Montana and North Dakota. But for Alabama and others accustomed to handling only trickles of mail ballots, a quick transition to voting by mail would be wrenching. Most of those states are in the Southeast, but not all: In New York, only 3.5 percent of ballots in the 2018 midterm election were absentee.

Still, for all the concerns about the impact of the virus on the presidential election, the six swing states most likely to determine the outcome in November — Michigan, Pennsylvania, Wisconsin, Florida, North Carolina and Arizona — all allow broad access to voting by mail.

The election this week in Wisconsin offers a cautionary example of what could happen to a state unprepared for a mostly mail election. In the 2016 presidential election, voters there cast some 145,000 absentee votes by mail; in Tuesday’s election, there were over a million. The state’s election officials regularly process high volumes of absentee ballots, but the last-minute cascade left them swamped, said Kenneth Mayer, a political science professor at the University of Wisconsin.

The coronavirus pandemic is giving some states a taste of what an all-mail election in November would look like. The National Conference of State Legislators has tracked legislation or executive orders that could potentially increase mail balloting in more than a dozen states, both Republican- and Democrat-led, in response to the health risks of voting at polling places. (This page, from the web site Ballotpedia, offers a more detailed account of all actions by states.)

Ohio is among the states that have already acted. It is planning an all-mail primary election on April 28 and is sending postcards to voters telling them how to obtain a ballot and a postage-paid return envelope. Georgia, Iowa and West Virginia are doing much the same. New York issued an executive order allowing absentee voting without an excuse for its upcoming primary, Alabama has temporarily lifted its absentee-voting restrictions, and Michigan has ordered that its primary election in May be conducted by absentee ballot as much as possible.

Some states are encountering resistance. In New Mexico, where 27 county clerks have petitioned the state Supreme Court to allow them to send mail ballots to voters for its primary, the state Republican Party has intervened, arguing that voting by mail encourages fraud. Missouri’s attorney general, Jay Ashcroft, has told county clerks that he does not have the power to allow “no excuse” absentee voting. And Republicans are opposing calls by Democrats and local election officials for all-mail voting in Arizona’s primary election — even though four in five voters already do so.