The US Postal Service has warned states 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.
Voters and lawmakers in several states are also complaining that some mail collection boxes are being removed.
Even as President Donald Trump rails against wide-scale 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 autumn 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 Mr DeJoy, a prominent Trump political donor who was recently appointed.
The back-and-forth comes amid a vigorous campaign by Mr Trump to sow doubts about mail-in voting as he faces a difficult fight for re-election against Democrat Joe Biden.
Although Mr Trump casts his own ballots by mail, he’s repeatedly criticised 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 kerbside mail boxes, which is how many will cast their ballots, have abruptly been removed in some states.