But severe bottlenecks in delivering the mail, and then in verifying and counting the ballots, led to lengthy delays in close elections in several states this year, highlighting the potential danger of political and legal bedlam if reforms aren't put in place before Nov. 3.
By choking off fresh funding, Trump appeared intent on sabotaging the process and making his warnings a self-fulfilling prophesy.
During an interview Thursday on Fox Business Channel, Trump said he would reject $3.5 billion in supplemental funding to help local election officials staff up for the election, and a broader $25 billion boost for the USPS, that Democrats sought in the now-stalled negotiations to help Americans in the current recession.
"They want $3.5 billion for the mail-in votes. Universal mail-in ballots. They want $25 billion, billion, for the Post Office. Now they need that money in order to make the Post Office work so it can take all of these millions and millions of ballots," Trump said, repeating his false claims that mail-in voting would be fraudulent.
"But if they don't get those two items that means you can't have universal mail-in voting because they're not equipped to have it," Trump added.
Much as he has urged businesses and schools to reopen _ and has largely ignored the resulting spike in deaths and infections _ Trump insisted it was safe for voters to line up and cast ballots inside schools, churches, town halls and other polling places.
"There's nothing wrong with getting out and voting. ... They voted during World War I and World War II," said Trump, who votes by mail in Florida.
Vice President Joe Biden, the presumptive Democratic nominee, told reporters that the president's effort to undermine the postal service was "pure Trump." Biden added: "He doesn't want an election."
House Speaker Nancy Pelosi, D-Calif., also pushed back, noting that the agency's bipartisan Board of Governors, which was appointed by Trump, had recommended the $25 billion boost. The money would be used not just to process mail ballots but to ensure that health precautions are in place to protect both voters and polling place workers.
"It's a health issue," Pelosi told reporters on Capitol Hill. "You shouldn't have to choose between your health and your ability to cast your vote."
Democrats in Congress have called for an investigation of Louis DeJoy, a mega-donor to the president before Trump named him postmaster general in May.
DeJoy has purged a number of experienced officials from the agency's executive ranks and instituted sweeping changes _ including reductions of overtime availability, restrictions on extra mail delivery trips, testing of new mail sorting and delivery policies at hundreds of post offices, and the reduction of the number and use of mail processing equipment _ that are widely blamed for an increase in delayed and undelivered mail, including ballots in states where primaries have taken place.
In a letter, Pelosi and 174 other House Democrats urged DeJoy to halt the changes. They said timely deliveries of medicine, paychecks and tax refunds are especially critical in the pandemic and expressed concerns that reduced service and a recent move to stop classifying all election mail as first class may impact voting in the fall.
"If implemented now, as the election approaches, this policy will cause further delays to election mail that will disenfranchise voters and put significant financial pressure on election jurisdictions," the Democrats wrote.
Senate Democrats sent a similar letter to DeJoy, expressing frustration with changes that seem broadly aimed at slowing mail delivery.
"Under normal circumstances, delayed mail is a major problem _ during a pandemic in the middle of a presidential election, it is catastrophic," they wrote.
Trump has claimed, perplexingly, that "absentee" voting is fine while mail-in voting is not, even though they are the same thing. He also has expressed confidence in just one state _ Florida, where he is an official resident and votes by mail _ and its ability to process mail ballots, if only because a political ally is in charge.
"Florida's got a great Republican governor," Trump said recently. "Florida's different from other states."
Trump's vow to block fresh funding for the postal service comes as Trump's campaign and the Republican National Committee have filed legal challenges to several states that have expanded access to mail ballots and early voting.
After Nevada approved universal mail-in ballot legislation, Trump called the vote by the state Legislature "an illegal late night coup" and tweeted "See you in Court!." Nevada is one of the states where Republicans have filed lawsuits.
Thus far, Republican leaders who were quick to reject the president's suggestion to delay the election, have been largely silent about his efforts to curtail mail-in voting, much to the chagrin of election watchdogs.
Trump is deliberately "throwing the election into chaos," said Wendy Weiser, a vice president at NYU's Brennan Center for Justice and founder of the Voting Rights and Elections Project.
"This is beyond inappropriate _ it is an attack on our entire constitutional system," she added. "The ball is now in Congress' court not to be complicit."
Negotiations between the White House and Democrats on another coronavirus relief package remain at a standstill, with Trump objecting to the $2 trillion in aid that Democrats have suggested as a compromise. No resolution appears imminent, and the Senate recessed Thursday until after Labor Day, Sept. 7.
Trump's opposition to aiding the postal service _ even after his negotiators, Treasury Secretary Steven T. Mnuchin and chief of staff Mark Meadows reportedly agreed in principle to $10 billion _ was a bigger sticking point than previously known, possibly even the primary reason talks broke down over measures to extend unemployment insurance, eviction protections and provide local governments with much needed funding.
Trump first made his motivations explicit during a news briefing Wednesday at the White House, stating outright that he would not approve $25 billion in emergency funding for the postal service, or $3.5 billion in supplemental funding for election resources, arguing that it costs too much.
“The bill is not going to happen ... because we can’t give them the kind of ridiculous things that they want that have nothing to do with the China virus,” Trump said. “So, therefore, they don’t have the money to do the universal mail-in voting. So, therefore they can’t do it, I guess. Right?”