The United States Postal Service (USPS) admitted that it could not meet a federal judge's deadline to check processing centers for undelivered mail-in ballots.

USPS said that by doing so, they would disrupt its Election Day operations. U.S. District Court Judge Emmet Sullivan in Washington ordered the checking Tuesday morning.

Sullivan released the order after USPS said its delivery performance had dwindled over the past few days. The USPS could not say whether over 300,000 ballots received in its respective facilities had been delivered.

USPS' sweep was set to be conducted in 12 postal districts, including battleground states such as Arizona, Florida, Georgia, Pennsylvania, Texas, and Wisconsin.

Sullivan gave USPS until 3:30 p.m. to finish the ordered task. The agency said it would continue its preplanned daily review process in its 220 facilities nationwide that process ballots and will try to deliver the remaining ballots.

Responding to the agency, the federal judge said he would let the process continue. However, he reprimanded the federal officials, saying they should be prepared to "discuss the apparent lack of compliance with the court's order." 

Related story: USPS to Apply 'Extraordinary Measures' to Deliver Mail-In Ballots on Time

The judge set the hearing on Wednesday. Many had questioned the credibility of mail-in voting, including President Donald Trump.

Trump even claimed that mail-in voting could be linked to election fraud.

Polls Closing

Mail-in ballots deadline had already passed in 29 states. There are still undelivered ballots in some states, including Miami-Dade County, Florida.

Princetown Post Office did a sweep in Miami and found 62 ballots. Prior to this year's election night, there were already claims and worries over USPS's ability to handle the delivery of mail-in ballots.

Mail-in Ballots

Timely delivery of the ballots has been one of the concerns. Voting rights activists are also worried about the loss of ballots, particularly in battleground states, as it could change the presidential election results. 

Related story: Black and Latino Voters Worry About Mail-In Voting

In 2016, one of every two ballots was rejected in Pennsylvania alone due to a missed deadline. Trump won over Hillary Clinton that year by 44,292 votes.

A USA Today report said that a missed deadline is second to the most common reason for rejected absentee ballots in the country. It is next to non-matching signatures.

In an earlier NPR report, over 500,000 ballots were rejected for the delay in this year's June primary election. NPR earlier reported that this might be a relatively small number. However, it could be crucial in a close election, particularly when voters are expected to cast absentee and mail-in ballots.

A political scientist at the Massachusetts Institute of Technology, Charles Stewart, said that those who use mail-in voting for the first time are more prone to be rejected. That is because of errors in casting a ballot.

"That's the sort of thing that makes me wary about what's going to happen in November when we get an even larger influx of people who haven't voted, or haven't voted by mail in the past," Stewart noted in an NPR report.