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For a long time, Americans believed that Election Day was simple. Polls opened in the morning. Ballots were cast. By nightfall, the country would begin to learn the outcome. Federal law seemed to reinforce that intuition. For more than a century, Congress has required that federal elections occur on “the Tuesday next after the first Monday in November.”

Watson v. RNC, now before the Supreme Court, asks whether that familiar rule might quietly disenfranchise thousands of voters.

The dispute arises from Mississippi’s practice of counting absentee ballots that arrive several days after Election Day so long as they were mailed on time, a method similar to that of dozens of states across the country. Challengers argue that federal law does not allow such flexibility. Congress established a single day for federal elections, they say, and ballots must therefore be received by that day. The 5th Circuit agreed, concluding that Mississippi’s rule conflicts with federal statutes governing the timing of federal elections.

At first glance, the argument sounds technical. In reality, the case raises a much more significant question about the structure of American democracy and who ultimately controls its rules. Critically, it has frightening echoes to a number of recent voting cases before the court, including a major one in which the justices rejected the same intellectual impulse that has been revived in Watson.

To see why, it is important to understand what Congress was trying to accomplish when it created a uniform Election Day. In the early 19th century, states often held elections weeks apart. That practice created obvious opportunities for political manipulation. Voters in later-voting states could react to results from earlier contests. Congress intervened in 1845 to establish a single national election date—a rule later extended to congressional elections—so that federal elections would occur simultaneously across the country.

Properly understood, the goal was coordination, not........

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