US President Donald Trump on Thursday floated an unprecedented delay to the 2020 election -- which polls show him losing -- but the suggestion drew cross-party fire and has virtually no chance of being accepted.

Trump, who faces Democrat Joe Biden on November 3, has no constitutional authority to change the date, which is set by law. His repeated airing of the idea, however, broke with presidential custom, adding to tension in a country ravaged by fallout from the coronavirus pandemic and set on edge by bitterly partisan politics.

Trump's rationale was his claim, not backed by evidence, that large-scale use of mail-in voting as a way to protect voters during the pandemic will lead to mass fraud.

Mail-in voting will make it "the most INACCURATE & FRAUDULENT Election in history. It will be a great embarrassment to the USA," he said in a tweet.

"Delay the Election until people can properly, securely and safely vote???" Trump asked.

Later at a press conference, he again sought to cast doubt on the entire election.

"Do I want to see a date change? No. But I don't want to see a crooked election. This election will be the most rigged election in history if that happens," he said.

Trump fired his initial tweet minutes after second quarter data showed a 32.9 percent annualized collapse in the economy compared to the first quarter, and down 9.5 percent compared to the same April-June period last year.

Economic pain and mass unemployment, together with what critics call incompetent management of the coronavirus crisis, is a big reason why Trump finds himself staring down the barrel of defeat in less than 100 days.

"Trump's threat is nothing more than a desperate attempt to distract," the Democratic National Committee said in a statement. "Trump can tweet all he wants, but the reality is that he can’t delay the election, and come November, voters will hold him accountable."

Trump's Republicans say no

Presidential elections are scheduled by law to take place on the first Tuesday after November 1, which this year is the 3rd. The United States has never allowed a delay, holding elections even during the Civil War, and only Congress could change the timing.

With Democrats ruling the House of Representatives and Trump's Republicans in the Senate swiftly voicing opposition, the idea will not get far.

"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," the Republican leader of the Senate, Mitch McConnell, told WNKY television.

Asked for a reaction, Republican Senator Marco Rubio, a Trump ally, said: "I wish he hadn't said that." The pandemic is still out of control across swaths of the United States, most lately hitting Republican-led states like Florida and Texas.

Given the virus' threat, there are widespread doubts over how many people will come out to vote and whether sufficient volunteers can be found to staff the polling stations. During the presidential primary contests earlier this year, several states delayed voting or opened fewer polling sites.

In addition, major sporting events have been cancelled or curtailed and there are serious doubts across much of the country over whether schools and universities will reopen for in-person classes in September.

Little evidence for fraud

But as the general election date approaches, Trump has angrily resisted Democratic-led calls for an increase in mail-in voting as a way to protect Americans from potential exposure to the COVID-19 virus in polling stations.

There is no evidence of meaningful fraud in US elections, including with the use of mail-in ballots, which are already common in some states.

This has led to mounting accusations that Trump, the first president to seek reelection after having been impeached, will not accept a defeat. He recently refused in an interview to guarantee that he would honour the results, another first for a president.

Catching the mood of Trump opponents ahead of the election, former president Barack Obama said Thursday at the funeral of civil rights hero John Lewis that few elections have been "as urgent on so many levels."

"Just everybody's got to come out and vote," he said in a fiery eulogy.