US president Barack Obama has criticised Donald Trump for sowing suspicion about the integrity of the election, telling the Republican presidential nominee to "stop whining" and focus on winning votes.
Mr Obama called Mr Trump's intensifying, pre-emptive warnings about voter fraud "unprecedented" in modern politics.
The rhetoric is not based on any evidence, Mr Obama said, but is simply aimed at discrediting the election before the first votes are counted.
"You start whining before the game's even over?" Mr Obama said at a press conference. "Then you don't have what it takes to be in this job."
Mr Obama delivered his biting rebuke at the White House, where he hosted Italy's prime minister, Matteo Renzi, for a state visit.
The president initially said he would pull his punches when it comes to politics, respecting the official setting. But when he was asked about Mr Trump's rhetoric, he did not hold back.
The president also accused Mr Trump of showering praise and modelling his policies on Russian president Vladimir Putin to a degree that is "unprecedented in American politics".
He said he was "surprised and troubled" by Republican politicians who he said are echoing their presidential nominee's positions on Russia.
Mr Trump has praised Mr Putin as a strong leader and criticised Mr Obama and the Democratic presidential nominee, Hillary Clinton, for Washington's deteriorating relationship with Moscow.
In an interview on Monday, Mr Trump said Russia "can't stand" either Democrat. Mr Trump promised a closer relationship with Mr Putin, if elected, starting with a possible meeting with Mr Putin before the US inauguration.
The president's remarks come as Mr Trump and his Republican allies are looking for ways to swing momentum their way after a damaging few weeks in the campaign.
Earlier on Tuesday, Republicans seized the latest developments in the Clinton email controversy as an issue that could help Mr Trump make up ground in the final presidential debate on Wednesday night.
That followed news that the State Department had asked FBI officials to lower the classification of a sensitive email related to the attack on the US compound in Benghazi, Libya.
FBI notes released on Monday revealed discussion of a "quid pro quo" in trying to get the email reclassified, though it's not clear who first raised the issue and both State and FBI officials deny any bargaining took place.
Mr Obama said the version Mr Trump and other Republicans are talking about is "just not true".
Mr Trump called it "one of the great miscarriages of justice" in history.
As the final debate has approached, the GOP candidate has continued his warnings that the election is "rigged," angering not only Democrats but fellow Republicans who worry his claims will hurt public faith in all elections.
And as news about the emails hit, Melania Trump made her first public comments about the allegations of sexual assault and misconduct lodged against her husband.
In an interview with Fox News, Mrs Trump said she believes the accusations were coordinated by political rivals: "They want to damage the presidency of my husband, and it was all planned, it was all organised from the opposition."
Her comments carried echoes of Mrs Clinton's allegations of a "vast right-wing conspiracy" organised to raise similar allegations against her husband two decades ago. Mr Trump notably tried to revive Bill Clinton's history by inviting his accusers to the last debate. His guest list for Wednesday's face-off in Las Vegas signalled he hoped to change the subject.
The Trump campaign said that Pat Smith, whose son, Sean Smith, was killed in the attack in Benghazi, will be attending the debate in Las Vegas as the candidate's guest.
Mrs Smith was a featured speaker at the Republican National Convention, where she delivered an emotional speech blaming Mrs Clinton for her son's death and accusing her of lying to families about what sparked it.
Mrs Clinton planned to spend Tuesday in New York preparing for the debate in Las Vegas. Mr Trump was due to hold rallies in Colorado.
The disclosure of FBI documents revives questions about Mrs Clinton's use of personal email during her time as secretary of state. The issue that has dogged her campaign and damaged voters' trust in her even as she remains the favourite ahead of the November 8 vote.
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