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.

Florida is a good example. It allows counties to begin verifying signatures on mail ballots and then feeding them into scanners 22 days before election day. Florida officials also quickly assemble the vote counts produced in the state’s early voting centers. Florida state law then requires county elections supervisors to report all early votes and tabulated mail ballots within 30 minutes of the polls closing. Recent research shows that in 2016, results from early voting and mail ballots in Florida were released more quickly than results from votes cast on election day.

Even if all states can’t produce a complete preliminary count of their ballots by the wee hours of Wednesday morning, and even if the earliest votes skew Republican, these earliest returns may still contain enough information to allow us to see whether Mr. Trump or Mr. Biden has a clear path to 270 electoral votes—and whose path is wider.