US President Donald Trump is using the impossible suggestion of delaying elections as a tool to scuttle plans by many states to allow all voters to cast their ballots by mail and to lay the basis for questioning the validity of the results.

After asking in yesterday morning's tweet, "delay the Election until people can properly, securely and safely vote???", he said at an evening news conference: "I don't want a delay. I want to have the election."

"But I also don't want to have to wait for three months and then find out that the ballots are all missing and the election doesn't mean anything," he said.

The President does not have the authority to delay the polls and the Constitution gives only the Congress the power to set the election day and the Democrats hold the majority in the House of Representatives.

The election day is fixed by an 1845 law as the first Tuesday in November, unless it falls on the first day of the month in which case it will be the next one.

Even in the Senate, which his Republican Party controls, Trump's suggestion was promptly shot down.

"We'll find a way to do that again this November 3" as scheduled, the leader of the Republican Majority in the Senate, Mitch McConnell, told a TV interview.

"Never in the history of the country, through wars, depressions and the Civil War, have we ever not had a federally scheduled election on time," he said.

Senator Kamala Harris, a front-runner to be the Democratic Party's vice-presidential candidate tweeted: "Donald Trump is terrified because he knows he's going to lose to @JoeBiden."

The issue of postal voting by all registered voters has, however, created a split along the party lines with Democrats supporting it while Republicans are sceptical.

Trump's tweet said: "With Universal Mail-In Voting (not Absentee Voting, which is good), 2020 will be the most INACCURATE & FRAUDULENT Election in history. It will be a great embarrassment to the USA."

He returned to the theme at the news conference criticizing the plan by California, the largest state, and six others to send out postal ballots to all voters this year instead of the earlier system under which voters would have had to request them and in many give a valid reason for not being able to vote in person.

Several other states are also considering extending postal ballots to all.

Trump said: "It's very, very unfair to our country. If they do this, our country will be a laughingstock all over the world because everyone knows it doesn't work."

He may have a point going by the experience with postal ballots that surged in recent primary elections as people wanted to avoid coming to voting booths because of the Covid-19 pandemic.

In New York state, for example, the Board of Elections has not been able to announce all the results even a month after the primaries on June 23 because of problems with postal ballots.

One of the candidates in the congressional primaries, Suraj Patel, said that about 25 per cent of the postal ballots had been rejected, most on technicalities. That may be costing him the election.

Even the Amazon CEO Jeff Bezos-owned Washington Post, which leans towards the Democratic Party, criticized it with a headline that called it "a train wreck and a warning to us all".

Trump cited such reports in the newspaper, although overall it ran articles favorable to universal postal ballots, to make his case questioning the ability of states to handle hundreds of millions of mail-in votes in a timely and safe manner.

When most voting is done in person at the polling booth results are known almost immediately after the close of polls.

The process becomes cumbersome if postal votes are the majority of votes cast as they will need to be verified individually to see if they meet various regulations and can be challenged by candidates' representatives and even end up in court.

In some states, postal ballots can be sent in 60 days before the November 3 election day, which means that in their cases the election campaign effectively ends on September 4, while in others it continues.

Trump also raised the threat of foreign countries like Russia or China manipulating the election results with a flood of forged votes.

The 2000 election in which Al Gore ran against George W. Bush was the last time there was a delay in knowing the results.

In the close race, the counting of paper votes was decided 35 days later after a final verdict by the Supreme Court following a series of legal challenges..