US President Donald Trump, lagging in the polls and facing economic turmoil amid an escalating health crisis, has suggested delaying this year’s American presidential election.

It was the first time Trump publicly raised the idea of pushing back the balloting – as he pushed unsubstantiated allegations mail-in voting due to the coronavirus pandemic would result in fraud.

Shifting the election is virtually impossible, but the mere suggestion of delay was extraordinary in a nation that has held itself up as a beacon to the world for its history of peaceful transfer of power, even during major crises.

It marks another bracing attempt by Trump to undermine confidence in the American political system.

The presidential election – held the Tuesday after the first Monday in November in every fourth year – is enshrined in US federal law and would require an act of Congress to change.

Lawmakers in both parties slammed the door shut on the notion Thursday (local time).

Regardless, the US constitution makes no provisions for a delay in the end of Trump's term: noon on January 20, 2021.

“With Universal Mail-In Voting (not Absentee Voting, which is good), 2020 will be the most INACCURATE & FRAUDULENT Election in history," Trump tweeted.

"It will be a great embarrassment to the USA. Delay the Election until people can properly, securely and safely vote???”

Only five US states conduct elections entirely by mail, although more states expect to rely more heavily on mail-in ballots in November because of the virus outbreak.

California has announced plans to send ballots to all registered voters for the fall election, but will also have in-person voting options available.

Trump's tweet came just minutes after the government reported the US economy shrank at a dizzying 32.9 per cent annual rate in the April-June quarter, by far the worst quarterly plunge ever, as the coronavirus outbreak shut down businesses, threw tens of millions out of work and sent unemployment surging to 14.7 per cent.

With just over three months until election day, Trump trails in the polls nationally and across battleground states, and some surveys even suggest traditionally Republican-leaning states could be in play.

While Trump has come back before after trailing consistently in the polls throughout 2016, the survey data has raised the possibility that he could face a landslide loss if he doesn’t turn things around.

Trump has increasingly sought to cast doubt on November’s election and the expected pandemic-induced surge in mail-in and absentee voting.

He has called remote voting options the “biggest risk” to his reelection.

His campaign and the Republican Party have sued to combat the practice, which was once a significant advantage for the GOP.

There is no evidence of widespread voter fraud through mail-in voting and the states that use it exclusively say they have necessary safeguards in place to ensure that a hostile foreign actor doesn’t disrupt the vote. Election security experts say that all forms of voter fraud are rare, including absentee balloting.

Most states are still finalising their plans for November. A small number of states sent ballots to voters during the primaries, but most states are not expected to do so in November. Instead, voters will have to request an absentee ballot if they want to vote at home.

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Voters and public health officials have expressed concerns about the potential dangers for spreading the virus during in-person voting, and states have reported difficulty filling poll worker positions given the pandemic.

Trump’s stated concern for poll safety defies his otherwise aggressive push to “reopen” the nation from partial shutdowns meant to slow the spread of the virus, even as rising confirmed coronavirus cases and deaths have pushed the US to the top of the list for the world outbreak.

Top Republicans in Congress quickly rebuffed Trump’s suggestion.

Senate Majority Leader Mitch McConnell said the election date is set in stone, while House GOP leader Kevin McCarthy said the election “should go forward” as planned.

Republican Senator Marco Rubio of Florida said the election will take place as planned, "be legitimate and...people should have confidence in it”.

Democratic House Speaker Nancy Pelosi replied to Trump’s tweet by tweeting a quote from the constitution assigning Congress the power to set the timing of elections.

One Republican governor, Chris Sununu of New Hampshire, also quickly shot down Trump's idea: “Make no mistake: the election will happen in New Hampshire on November 3.

“End of story. Our voting system in New Hampshire is secure, safe, and reliable. We have done it right 100 per cent of the time for 100 years – this year will be no different."

Hogan Gidley, the Trump campaign's national press secretary, pointed to the delays in counting votes in New York's primary.

“The President is just raising a question about the chaos Democrats have created with their insistence on all mail-in voting. They are using coronavirus as their means to try to institute universal mail-in voting, which means sending every registered voter a ballot whether they asked for one or not."

Trump refused in an interview just weeks ago with Fox News to commit to accept the results of the upcoming White House election, recalling a similar threat he made weeks before the 2016 vote.

“I have to see. Look ... I have to see,” Trump told moderator Chris Wallace during a wide-ranging interview on Fox News Sunday.

“No, I’m not going to just say ‘yes.’ I’m not going to say ‘no,’ and I didn’t last time, either.”

Last month, he also said in Arizona: “This will be, in my opinion, the most corrupt election in the history of our country.”

Trump and many members of his administration have previously availed themselves of absentee voting, but Trump has sought to differentiate that from a growing push by states to mail all registered voters either ballots or absentee request forms.

“He has absolutely no power to do this," said Wendy Weiser, director of the democracy program at the Brennan Center for Justice at New York University’s law school. "This is just sowing chaos.”

Democrats have pushed to include billions of dollars in the next coronavirus relief bill to fund election security and accessibility improvements for this year's vote, but Trump and Republicans have so far resisted those efforts.

Just months ago, in April, Trump had ruled out the prospect of trying to change the election after Democratic rival Joe Biden predicted Trump would do so.

“I never even thought of changing the date of the election," he said. "Why would I do that? November 3. It’s a good number. No, I look forward to that election. I’m not thinking about it at all," he added.

“Not at all.”

And in March, Trump opposed moves by several states to delay their presidential primaries because of the coronavirus.