Nate Persily and Charles Stewart in the WSJ:

Pundits are warning that election night in November may turn into election week or even election month. Amid the pandemic, election officials are bracing for a flood of ballots sent by mail, and Americans may need to wait an unusually long time to know for sure who won and by how much. But that doesn’t mean we will be in the dark about the next president until all the official state counts are completed. In all likelihood, we will have a good idea on election night, or within a few days after, of whether Donald Trump or Joe Biden won the White House.

Many observers expect the states to take an inordinately long time to count votes this November because, according to a recent nationwide survey, more than 50% of voters may end up casting mail ballots this year, up from 20% in 2016. Thirteen states, including the swing states of Michigan, New Hampshire, Pennsylvania and Wisconsin, wait until election day to “process” their mail ballots. (Processing is the act of verifying the identity of the voter who returned the mail ballot, usually by matching the signature on the ballot to one on file; the actual counting comes later.) If the election comes down to mail ballots cast in those states, waiting for them to verify the ballots and count the votes could push the election into overtime.

Because more Democrats than Republicans have requested mail ballots this year, the vote totals on Election Day—reflecting all in-person voting but not all mail ballots—could paint a misleading picture about who won. Josh Mendelsohn, CEO of the Democratic data firm Hawkfish, has warned of an election-day “red mirage” of victory for Mr. Trump, which will be replaced in short order by a “blue shift” as the outstanding, heavily Democratic mail ballots are counted. Such a dramatic change from the election-night result could lead to baseless Republican charges of fraud and cries that the election was rigged, which could spark dangerous political unrest.

These nightmare scenarios ignore several key facts, however. Most states begin processing their ballots before election day, and almost all begin putting them through scanners before the polls close. Many states intermingle sent-by-mail and election-day ballots at the polling places, where they are scanned together, so that when the precinct count is released, it contains both in-person and mail ballots. In such states—which include such battlegrounds as New Hampshire and most of Wisconsin—the polling place counts may be released a few hours later than they might in another year, but not days later. More generally, local jurisdictions have been preparing all summer for a surge in mail ballots; most will be counted on election day in parallel with the day’s in-person ballots, so that the results of many ballots cast in advance can be announced early on election night.