US President Donald Trump has been recorded telling Georgia's top election official to "find" enough votes to overturn the election result.
"I just want to find 11,780 votes," Mr Trump told Republican Secretary of State Brad Raffensperger in a recording released by the Washington Post.
Mr Raffensperger is heard replying that Georgia's results are correct.
Joe Biden won Georgia alongside other swing states, winning 306 electoral college votes to Mr Trump's 232.
Vice-President-elect Kamala Harris called Mr Trump's comments "a bold abuse of power".
It comes ahead of two crucial runoff elections in Georgia on Tuesday that will decide which party controls the Senate.
Since the 3 November vote, Mr Trump has been making unsubstantiated allegations of widespread electoral fraud.
All 50 states have certified the election result, some after recounts and legal appeals.
Congress is due to formally approve the election result on 6 January and Mr Biden, a Democrat, is due to be inaugurated as president on 20 January.
All 10 living former US defence secretaries have urged President Trump not to question the election results in an opinion piece for the Washington Post. The group also said he should not involve the military in voting disputes, an idea that's been mooted by some of Mr Trump's supporters.
What happened during the Trump call?
In excerpts of Saturday's phone call released by the Washington Post, Mr Trump can be heard alternately cajoling and pressurising Georgia's secretary of state.
He insisted that he had won the election in Georgia and told Mr Raffensperger that there was "nothing wrong with saying you have recalculated".
Mr Raffensperger responded by saying: "The challenge you have, Mr President, is that the data you have is wrong."
Later in the call, Mr Trump said the rumour was that ballots had been shredded and voting machinery had been removed from Fulton County in the state - claims denied by Mr Raffensperger's lawyer.
The president then threatened the official with possible legal consequences.
"You know what they did and you're not reporting it. That's a criminal offence. You can't let that happen. That's a big risk to you and to Ryan, your lawyer," Mr Trump said.
He then called for the extra 11,780 votes - which would have given him a total of 2,473,634 votes in the state, one more than Mr Biden, who received 2,473,633 votes.
He told Mr Raffensperger he should re-examine the result in the state.
"You can re-examine it, but re-examine it with people who want to find answers, not people who don't want to find answers," he said.
"Mr President, you have people who submit information and we have our people that submit information, and then it comes before the court and the court has to make a determination," Mr Raffensperger replied. "We have to stand by our numbers, we believe our numbers are right."
Mr Trump also warned Mr Raffensperger that by refusing to recalculate the election result he would deter Republicans from turning out to vote in Tuesday's runoff elections for the Senate.
If the two Democratic contenders win, then there will be equal numbers of Republican and Democratic senators, and Kamala Harris, as vice-president-elect, will have the deciding vote.
Mr Biden's Democrats already control the lower House of Representatives.
Both President Trump and President-elect Biden are due to visit Georgia on Monday to campaign ahead of the elections.
On Sunday Mr Trump tweeted that Mr Raffensperger had not given details of the fraud the president alleges. "He has no clue!" the president tweeted.
Mr Raffensperger tweeted back: "Respectfully, President Trump: What you're saying is not true. The truth will come out."
Respectfully, President Trump: What you're saying is not true. The truth will come out https://t.co/ViYjTSeRcC— GA Secretary of State Brad Raffensperger (@GaSecofState) January 3, 2021
The White House has not commented on the release of the audio.
The only Democrat on Georgia's state election board called on Mr Raffensperger to launch an investigation into the phone call.
"It's a crime to solicit election fraud and asking the secretary to change the votes is a textbook definition of election fraud," David Worley, an Atlanta lawyer, said in an interview with the Washington Post.
Senate Minority Whip Dick Durbin, a Democrat, said Mr Trump was "unhinged and dangerous" and that his call to Mr Raffensperger merited "nothing less than a criminal investigation".
Moderate Republican Adam Kinzinger also tweeted: "This is absolutely appalling. To every member of Congress considering objecting to the election results, you cannot - in light of this - do so with a clean conscience."
In the final weeks of his presidency, Donald Trump is struggling for ways to hold on to power. His latest act was to berate, beg and threaten Georgia's secretary of state to "find" the votes necessary to move the state to his column.
Never mind that there has been no credible evidence of substantive fraud. Never mind that Georgia has counted, then recounted, then recounted again the ballots and found no major disparities. Never mind that the Republican-controlled state has certified its election results and, even if Georgia flipped, it wouldn't be enough to re-elect Trump.
The tenor of Trump's call suggests he's not just manoeuvring for post-presidential power or trying to boost his fund-raising haul, however. He appears to firmly believe he won and is willing to use all the tools, some of questionable legality, to push state officials to shift the election in his favour.
The call is reminiscent of Trump's "I would like you to do us a favour, though" conversation with Ukrainian President Volodymyr Zelinsky, where he pressured and cajoled a foreign leader for political help.
That contact resulted in Trump's impeachment. The brazenness of Trump's Saturday call may make it more difficult for the president to rally political support to his cause.
The president's desperate times have led to the most desperate of measures.
When will the election result be certified?
There is a developing split in the Republican party after 11 senators said they would not vote to certify Mr Biden's election victory in a Senate session on 6 January.
Ted Cruz led the group of senators, calling for a 10-day delay to audit unsubstantiated allegations of election fraud.
Vice-President Mike Pence - who as president of the Senate is due to oversee the session and declare Mr Biden the winner - said he welcomed the move.
He stopped short of repeating allegations of fraud but his chief of staff said Mr Pence shared what he called "the concerns of millions of Americans about voter fraud and irregularities".
Meanwhile, four Republican senators including Mitt Romney have signed a statement saying they will vote to certify Mr Biden's win.
Senate Majority Leader Mitch McConnell has already recognised Mr Biden's victory and asked other Republicans not to object.
A group of Republicans in the lower chamber of Congress, the House of Representatives, is also planning to contest the election results.
However, for an objection to be upheld, a majority in both chambers must vote in favour. Republicans hold the majority in the Senate but some of their number have already said they will not contest the results. Democrats are in the majority in the House.
On Sunday a new Congress convened for the first time, with senators and representatives sworn in. Democrat Nancy Pelosi was narrowly re-elected as House Speaker.