The final debate between the main candidates takes place on Wednesday
Barack Obama has told Donald Trump he does not have what it takes to be president if he is already "whining" before the vote has taken place.
The president tore into the Republican nominee following his continued claims that November's US election is rigged.
"If you start whining before the game's even over; if whenever things are going badly for you and you lose, you start blaming somebody else, then you don't have what it takes to be in this job," Mr Obama said during a press conference at the White House.
He called Mr Trump's intensifying warnings about voter fraud "unprecedented" in modern politics, saying his claims were not based on evidence, but simply aimed at discrediting the election.
"There is no evidence that that has happened in the past or that there are instances in which that will happen this time," Mr Obama said.
"And so I'd invite Mr Trump to stop whining and go try to make his case to get votes."
He also accused the Republican of modelling his policies on Russian President Vladimir Putin.
Mr Trump has praised Mr Putin as a strong leader and criticised Mr Obama and Democratic presidential nominee Hillary Clinton for Washington's deteriorating relationship with Moscow.
He has promised a closer relationship with Mr Putin if elected, starting with a possible meeting before the US inauguration.
Mr Obama's remarks came as Mr Trump and his Republican allies look for ways to swing momentum their way after a damaging few weeks in which his campaign has been rocked by allegations of sexual assault.
Mr Trump has denied the claims, saying they are "outright lies".
He has claimed the media has "slandered and lied about me" and accused the press of conspiring with Hillary Clinton against his White House bid.
Meanwhile, Vogue magazine has backed Mrs Clinton to become the first ever female president.
Despite having no history of political endorsements, the fashion magazine said that "given the profound stakes" of the 2016 election "and the history that stands to be made, we feel that should change".
In an article endorsing Mrs Clinton, the magazine said: "For all the chaos and unpredictability and the sometimes appalling spectacle of this election season, the question of which candidate actually deserves to be President has never been a difficult one."
It added: "We understand that Clinton has not always been a perfect candidate, yet her fierce intelligence and considerable experience are reflected in policies and positions that are clear, sound, and hopeful."