U.S. Sens. Tammy Baldwin and Ron Johnson are calling for an investigation into missing absentee ballots in Wisconsin.

"In light of the COVID-19 pandemic, there was an unprecedented increase in requests for absentee ballots, as more and more voters sought to comply with public health guidance and avoid potential risk to their health by avoiding in-person voting," they wrote Thursday in a letter to Inspector General of the U.S. Postal Service Tammy Whitcomb. "The United States Postal Service (USPS) in short, had an out-sized role in ensuring Wisconsinites could safely exercise the right to vote and participate in our democracy."

Baldwin and Johnson cited a Milwaukee Journal Sentinel report from Wednesday about missing absentee ballots, which detailed state officials reporting three tubs of them from Oshkosh and Appleton discovered in a mail processing center. The Milwaukee Election Commission has also called for an investigation into a separate set of undelivered ballots. Problems were also reported in Fox Point.

"Unfortunately, there have been numerous accounts from the state that USPS failed to fulfill that critical function for some voters," the senators wrote. "We are concerned there may be more examples, and request that you promptly open an investigation to determine the cause of these failures, which appear to have disenfranchised many Wisconsin voters."

Baldwin is a Democrat from Madison. Johnson is a Republican from Oshkosh.

Would-be voters across the state have expressed a host of frustrations about trying to obtain absentee ballots so they could avoid going to the polls during the coronavirus pandemic. Many have said ballots they requested long ago did not arrive by Tuesday, the deadline for getting their ballots postmarked.

Robert Sheehan, a Wisconsin media contact for the Postal Service, said Thursday that the Postal Service is investigating reports that ballots were processed for mailing to clerks without being postmarked. A U.S. Supreme Court ruling late Monday said absentee ballots had to show a postmark by Tuesday to be counted. Sheehan declined to provide further details.

"As the COVID-19 crisis continues and as more voters are likely to request to vote by mail where available, this year's forthcoming elections will require that USPS's existing vote-by-mail procedures are strictly and effectively followed," the senators wrote. "It is critical that you quickly identify what has gone wrong and propose solutions that USPS can swiftly implement."

Journal Sentinel reporters Laura Schulte and Alison Dirr contributed.

Contact Mary Spicuzza at (414) 224-2324 or mary.spicuzza@jrn.com. Follow her on Twitter at @MSpicuzzaMJS.