In a stunning Election Day action, the U.S. Postal Service rejected a federal judge’s order to carry out a special sweep of mail facilities to hunt for more than 300,000 ballots that somehow went missing.

“U.S. District Judge Emmet G. Sullivan … had given the mail agency until 3:30 p.m. [ET, Tuesday] to conduct the ‘all clear’ checks to ensure that any found ballots could be delivered before polls closed,” reported the Washington Post.

Sullivan’s order affected 12 postal districts serving 15 states.

“All clear” sweeps, designed to locate misplaced political and election mail, have been going on in mail facilities since January — but this time, it appears, they failed in a big way.

Justice Department attorneys representing the Postal Service sent a filing to Sullivan’s D.C. court just before 5 p.m., saying “the agency would not abide by the order,” the Post said.

The USPS chose instead to stick with its own regular inspections — meaning some ballots might not be located in time to be counted. And in this case, “some” may not be a trivial number.

The USPS disclosed Tuesday morning “that 300,523 ballots nationwide had gotten incoming scans at postal processing plants but not exit scans, leaving voting rights advocates worried that hundreds of thousands of votes could be trapped in the mail system,” the newspaper said.

Attorney Allison Zieve of the NAACP, which brought the lawsuit against the Postal Service with a group of voters and other civil rights groups, called the situation “super frustrating.”

All this is playing out against a distinct slowdown in the mail in many parts of the country, including key election battleground states.

“Voting and postal experts say the mail agency should be able to process 97% of incoming ballots,” the Post says. “But data shows the Postal Service missed that mark seven out of eight days. And in the past five days, processing scores dropped from 97.1% on Oct. 28, to 89.6% on Nov. 2.”

It gets worse: in 17 postal districts that cover 151 electoral votes, Monday’s on-time processing rate was even lower: 81.1%.

More than 65 million Americans have voted in this election using absentee ballots, and more than 27 million mail ballots remain outstanding, the Post says, citing the United States Elections Project.

Some of the 300,000 missing ballots were almost certainly requested but not used, when voters changed their minds and went to the polls in person — or chose not to vote at all.

But what of the rest?