MILWAUKEE (AP) U.S. Sens. Tammy Baldwin and Ron Johnson, in rare bipartisan unity, are calling on the U.S. Postal Service to investigate what happened to missing absentee ballots in Wisconsin.

In a letter to the Inspector General the senators cite numerous accounts from the state that the Postal Service failed to fulfill its critical mission of getting all those ballots in the hands of voters.

The Wisconsin Elections Commission reported Thursday that nearly 1.3 million absentee ballots had been requested by voters and about 235,000 ballots had not yet been returned as of Thursday.

What’s not known is how many of the outstanding ballots were completed by voters and mailed back by the Tuesday deadline.

State officials reported three tubs of ballots from Oshkosh and Appleton were discovered in a mail processing center. The Milwaukee Election Commission has also called for an investigation into a separate set of undelivered ballots.

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