WASHINGTON -- U.S. President Donald Trump, lagging in the polls and grappling with deepening economic and public health crises, on Thursday floated the startling idea of delaying the Nov. 3 presidential election. His campaign to sow doubt about the election's outcome drew immediate pushback from Democrats and Republicans alike in a nation that has held itself up as a beacon to the world for its history of peaceful transfer of power.

Trump suggested the delay as he pushed unsubstantiated allegations that increased mail-in voting due to the coronavirus pandemic would result in fraud. But shifting Election Day is virtually impossible and the very idea represented another bracing attempt by Trump to undermine confidence in the American political system.

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

Top Republicans in Congress quickly rebuffed Trump's suggestion. Senate Majority Leader Mitch McConnell said the election date is set in stone and House GOP leader Kevin McCarthy said the election "should go forward" as planned. Regardless, the Constitution makes no provisions for a delay in the end of Trump's term -- noon on Jan. 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 Thursday. "It will be a great embarrassment to the USA. Delay the Election until people can properly, securely and safely vote???"

After facing blowback from Republicans for even floating a delay, Trump appeared to retreat on Twitter Thursday afternoon, suggesting he was merely trying to highlight alleged problems with mail-in balloting. "Do I want to see a date changed, no," Trump said later during a press conference on the coronavirus response. "But I don't want to see a crooked election."

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.

In fact, only five 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. Experts assess that delays in counting mail-in ballots could mean results won't be known on Election Day.

Trump's suggestion of delaying the vote came just minutes after the government reported that the U.S. economy shrank at a dizzying 32.9% 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%.

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.

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 voter fraud is rare in all forms of balloting, including by mail.

So far, at least six states have confirmed that they will send mail ballot request forms to voters, according to the Brennan Center for Justice at NYU's Law School. New Mexico has passed legislation so that county clerks may send such forms to voters. California and Vermont will mail ballots to all active registered voters. In roughly three dozen states and the District of Columbia, officials may not send request forms or ballots to all voters for November.

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.

Speaking at Rep. John Lewis's funeral in Atlanta, former President Barack Obama implicitly addressed his successor's policies on voting.

"There are those in power doing their darnedest to discourage people from voting by closing polling locations and targeting minorities and students with restrictive ID laws and attacking our voting rights with surgical precision, even undermining the postal service in the run up to an election that's going to be dependent on mail-in ballots so people don't get sick," Obama said.

In an evening fundraiser with top Black congressional leaders, former Vice-President Joe Biden, the presumptive Democratic presidential nominee, blasted Trump's suggestion of a delay, calling it another example of a strategy to "stoke division and chaos" because Trump doesn't "want to focus on what's going on."

"This has got to be a wakeup call," Biden said. "It's time to respond with purpose, action and commitment. Because this election isn't just about voting against Donald Trump. It's about working to understand people's struggles. It's about all our families."

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.

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. 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.

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 U.S. to the top of the list for the world outbreak.

Trump has said the upcoming vote will be "the most corrupt election" in U.S. history and has refused to commit to accept the results, recalling a similar threat he made weeks before the 2016 election.

In April, Trump had ruled out the prospect of trying to change the election after Biden predicted Trump would do so. "I'm not thinking about it at all," Trump said. "Not at all."

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

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Associated Press writers Christina Cassidy and Bill Barrow in Atlanta, Holly Ramer in Hopkinton, New Hampshire, Emily Swanson and Mark Sherman contributed to this report.