The U.S. Postal Service has adopted a new rule that could create doubt about whether some ballots mailed by voters by Election Day will receive postmarks in time to be counted.

A USPS rule that took effect on Dec. 24 says mail might not receive a postmark on the same day the agency takes possession of it. The postal service says it isn’t changing its existing postmark practices and is merely clarifying its policy, but some election officials have looked to postmarks as a guarantee that mail ballots were cast before polls closed.

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The new rule holds implications for 14 states and Washington, D.C., that count ballots arriving after Election Day if they are postmarked on or before that day — commonly called a “ballot grace period.” In these states, ballots placed in the mail by voters before the deadline may not be counted if the postal service applies a postmark after Election Day.

The USPS rule says that “the postmark date does not necessarily indicate the first day that the Postal Service had possession of the mailpiece.”

The USPS rule comes as the U.S. Supreme Court prepares to consider a case that could eliminate ballot grace periods nationwide. The court’s decision, expected late this spring or next summer, could render the issues raised by the postmark rule moot.

Mail-in voting surged in 2020’s general election amid the COVID-19 pandemic, when 43 percent of voters cast their votes by mail. The percentage of voters mailing their ballots has fallen from that peak but remains above pre-pandemic levels. About 30 percent of voters cast mail ballots in 2024, according to data gathered by the U.S. Election Assistance Commission.

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While the vast majority of mail ballots were successfully cast last year, hundreds of thousands weren’t counted. During the 2024 election, 584,463 mail ballots returned by voters were rejected by election officials — 1.2 percent of returned mail ballots. About 18 percent of those ballots were rejected because they didn’t arrive on time.

The USPS defended the change in a lengthy response to criticisms published in the Federal Register. The agency emphasized that it does not administer elections and doesn’t advocate for or against voting by mail.

The postal service repeated its advice that voters mail their completed ballots at least a week before Election Day. And it noted that voters may request a manual postmark at their local post office free of charge.

“If customers are aware that the postmark date may not align with the date on which the Postal Service first accepted possession of a mailpiece, they will be better equipped to adjust their plans accordingly,” the response reads.

“And if policymakers or other entities that create rules utilizing the postmark date are aware of what the postmark date signifies, they are better equipped to determine whether their rules adequately serve their purposes.”