Three US states are suing the Trump administration over changes to the US Postal Service (USPS) ahead of the November election.
It is the latest development in a huge row over the federal agency, which is tasked with handling mail-in ballots for the vote.
The filing alleges “partisan meddling” is slowing deliveries, which could “endanger” election plans.
Neither the White House nor the USPS have yet commented on the filing.
Democrats allege that the head of the USPS, who was appointed by US President Donald Trump, has tried to deliberately “sabotage” the election with a series of recent reforms.
Postmaster General Louis DeJoy has dismissed the allegations. The top Republican donor told senators on Friday that all ballots will arrive on time and announced the reforms would be delayed until after the election.
But he has overseen the removal of mail boxes and reduced overtime pay for workers.
An unprecedented number of US voters are expected to vote by post this year due to the ongoing coronavirus pandemic.
Mr Trump has repeatedly made allegations that mail-in ballots will lead to voter fraud – something dismissed by experts and voting officials.
Hawaii, New Jersey and New York states are all plaintiffs in the lawsuit, as are the cities of San Francisco and New York.
“Thanks to a series of sweeping policy changes beleaguered by partisan meddling, the independent agency has become a political football set to undermine a federal election,” the filing reads.
It claims President Trump has “repeatedly, emphatically, and openly attempted to undermine mail-in voting efforts”.
Officials appointed by him meanwhile “eliminated or substantially altered” processes that kept mail delivered on time, leading to severe delays and a massive backlog.
New York state Attorney General Letitia James tweeted on Tuesday about filing the suit. “The slowdown is nothing more than a voter suppression tactic,” she wrote.
Last week Maryland’s attorney general said more than a dozen states were considering legal action against the Trump administration.
The US House of Representatives meanwhile has passed a bill for $25bn (£19bn) in funding for the USPS ahead of November’s election. The head of the Republican-led Senate however has said the body will “absolutely not pass” the bill into law.
At the Republican National Convention on Monday Mr Trump again claimed that mail-in ballots could lead to fraud. In his speech he called it “the greatest scam in the history of politics”.