Congressional Democrats have called for an investigation into decisions made by the head of the US Postal Service (USPS), which they say have slowed deliveries ahead of the election.
There is expected to be a huge rise in mail-in voting in November’s presidential vote, due to the coronavirus pandemic.
Democrats have suggested cost-cutting at USPS could affect the vote count.
But postal chief Louis DeJoy has insisted standards will be met.
“Although there will likely be an unprecedented increase in election mail volume due to the pandemic, the Postal Service has ample capacity to deliver all election mail securely and on-time in accordance with our delivery standards, and we will do so,” the Trump supporter told a board meeting on Friday.
But he said election officials had to “take our normal processing and delivery standards into account”.
Top Democrats including Senator Elizabeth Warren on Friday called on the USPS inspector general to investigate operational changes made by Mr DeJoy, including preventing postal workers from working overtime to deliver mail.
“Given the ongoing concerns about the adverse impacts of Trump Administration policies on the quality and efficiency of the Postal Service, we ask that you conduct an audit of all operational changes put in place by Mr DeJoy and other Trump Administration officials in 2020,” they said.
They asked for the inquiry to specifically focus on how this could affect election-related mail.
President Donald Trump has suggested that increased postal voting in November could lead to fraud and inaccurate results, There is little evidence to support his claims.
Mr DeJoy insisted on Friday that USPS was “not slowing down election mail or any other mail”.
While he had “good relationship” with Mr Trump, “the notion that I would ever make decisions concerning the Postal Service at the direction of the president, or anyone else in the administration, is wholly off-base”, he said.
A spokesperson for the Postal Service said it would welcome an investigation by the inspector general into the changes being made to make the service “more efficient”.
Primary elections in New York city on 23 June were hit hard by counting delays after a surge in mail-in ballots, leaving two congressional races without a winner for several weeks.
That vote has been seen by some as a warning of what could happen in the November election.
The US Postal Service lost $2.2bn (£1.7bn) in the three months to June and is in a “dire” financial position, said Mr DeJoy, who also on Friday released details of an overhaul of the agency’s leadership, including a management hiring freeze.
The Republican donor took over USPS in June and is the first postmaster general in nearly 20 years to be appointed from outside the agency, the Associated Press reports.
Changes he has made to cut costs threaten “the timely delivery of mail - including medicines for seniors, paychecks for workers, and absentee ballots for voters,” Ms Pelosi and top Senate Democrat Chuck Schumer said on Thursday.
The pair met with Mr DeJoy and White House officials on Wednesday in a meeting that Mr Schumer described as “heated”.
According to the Washington Post, the USPS leadership shake-up reassigns or displaces 23 top executives.