The top election official in the key state of Pennsylvania has warned voters to stop sending ballots through the post amid looming legal fights over election deadlines.
In a record early voting surge 70 million Americans have already cast ballots, more than half the total number who voted in the 2016 election. Most have done so by post.
But Kathy Boockvar, Pennsylvania’s secretary of state, urged voters to cease "putting their ballots in the mail," and told them to drop them off in person.
Republicans have gone to the US Supreme Court in an attempt to overturn a Pennsylvania rule that postal ballots arriving up to three days after Election Day could still be counted.
On Wednesday night the Supreme Court decided not to consider the Pennsylvania case before the election. However, the court rule on it in the days immediately after Election Day.
Meanwhile, Donald Trump blamed the Democrat leadership in Pennsylvania for violence that followed the recent fatal police shooting of a black man, Walter Wallace Jr, in Philadelphia.
Mr Trump, campaigning in Las Vegas, said: "You can't let that go on. Again, a Democrat-run state, a Democrat-run city, Philadelphia."
Joe Biden also denounced the violence, saying: "There is no excuse whatsoever for the looting."
Mr Biden on Wednesday night voted early in his home town of Wilmington, Delaware.
In a host of legal fights over postal ballots across the US, Democrats are attempting to expand the number that can be counted as more of their supporters traditionally vote in that way.
Republicans are trying to limit the number of postal ballots, with their lawyers often citing an increased potential for fraud.
More than 300 cases have been filed in 44 states, and in Washington DC.
In a similar case to that of Pennsylvania, Republicans in North Carolina objected to a ruling by the local election board that postal ballots can arrive up to nine days late.
That could delay the result in a close battleground state by over a week. The Supreme Court on Wednesday night declined to block the ruling, in a blow to President Trump. All ballots received before November 12 will be considered legitimate.
In an encouraging signal for Republicans the court issued a late night ruling this week rejecting an appeal brought by Democrats in Wisconsin, asking for postal ballots arriving up to six days late to be counted. The court said they must arrive on Nov 3, by 8pm.
Mr Trump said: "Hopefully the few states remaining that want to take a lot of time after November 3 to count ballots, that won't be allowed by the various courts. We're in courts on that."
He added: "We just had a big victory yesterday in Wisconsin on that matter."
The granular detail of the issues being fought over is illustrated by a case in Nevada, where Mr Trump's campaign is suing to stop the counting of absentee ballots in Las Vegas.
Lawyers for his campaign object to election observers being asked to stand 25ft away, arguing they won't be able to see computer screens used by those doing the counting.
Other battles are being fought over drop-off ballot boxes stationed around states for people to put their postal votes in.
In Texas a court rejected a challenge against Republican governor Greg Abbott's decision to have only one box in each county.
That means there will be only one box for 4.7 million people living in and around Houston.
In Michigan a judge blocked an order from the Democrat secretary of state, meaning voters will be able to openly carry guns at polling stations.
With less than a week left experts predicted an overall voter participation rate of about 65 per cent, which would be the highest since 1908.
But they also warned over one million postal ballots may end up not counting due to errors, which could affect the result of close races in some states.
Daniel Smith, a professor of political science at the University of Florida, said: "The vote-by-mail ballot rejections are going to be the 'hanging chads' of 2020."
He was referring to an election-defining dispute over ballots in Florida in the 2000 election between George W. Bush and Al Gore.
It is predicted around 70 million people will end up voting by post.
Rejection rates for postal ballots are usually about one per cent.
But that could be higher in states that have not previously dealt with large numbers of such votes, including Pennsylvania, North Carolina and Nevada.
The winning margin in some states could be less than the number of rejected postal ballots.
Rates for discounted postal ballots are usually highest among Hispanic, black, and young voters, meaning it could benefit Mr Trump who generally performs worse than Mr Biden among those categories.