The US Postal Service is warning states coast to coast that it cannot guarantee all ballots cast by mail for the November election will arrive in time to be counted, even if mailed by state deadlines, raising the possibility that millions of voters could be disenfranchised.
Voters and lawmakers in several states are also complaining that some curbside mail collection boxes are being removed.
Even as President Donald Trump rails against widescale voting by mail, the post office is bracing for an unprecedented number of mail-in ballots as a result of the coronavirus pandemic.
The warning letters sent to states raise the possibility that many Americans eligible for mail-in ballots this fall will not have them counted.
But that is not the intent, Postmaster General Louis DeJoy said in his own letter to Democratic congressional leaders.
The post office is merely "asking elected officials and voters to realistically consider how the mail works, and be mindful of our delivery standards, in order to provide voters ample time to cast ballots through the mail," wrote DeJoy, a prominent Trump political donor who was recently appointed.
The back-and-forth comes amid a vigorous campaign by Trump to sow doubts about mail-in voting as he faces a difficult fight for reelection against Democrat Joe Biden.
Though Trump casts his own ballots by mail, he's repeatedly criticized efforts to allow more people to do so, which he argues without evidence will lead to increased voter fraud that could cost him the election.
Meanwhile, members of Congress from both parties have voiced concerns that curbside mail boxes, which is how many will cast their ballots, have abruptly been removed in some states.
The Post Office released letters it sent to all 50 states and the District Columbia on its website.
While some states with permissive vote-by- mail laws were given a less stringent warning, the majority with more restrictive requirements that limit when a ballot must be cast were given a more dire warning.
The laws, the letter said, create a "risk that ballots requested near the deadline under state law will not be returned by mail in time to be counted." Many state officials criticised the move.