Trump and Democratic rival Hillary Clinton are set for their final presidential debate Wednesday, with three weeks to go before Americans head to the polls on November 8.
It is seen as a last chance for the Republican nominee, dogged by accusations of sexual misconduct and sinking poll numbers, to make his mark on millions of voters.
With the provocative billionaire's campaign reeling, Trump is likely to engage in more scorched-earth tactics if recent history is any guide.
But with Trump pressing the dangerous conspiracy theory that the US election is "rigged," Obama abandoned diplomatic decorum and skewered the mogul from the Rose Garden in front of visiting Italian Prime Minister Matteo Renzi.
"I'd advise Mr Trump to stop whining, and go try to make his case to get votes."
"I have never seen in my lifetime, or in modern political history, any presidential candidate trying to discredit the elections and the election process before votes have even taken place. It's unprecedented," Obama told a joint press conference.
"That is both irresponsible -- and, by the way, it doesn't really show the kind of leadership and toughness that you'd want out of a president. 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," he added.
"I'd advise Mr Trump to stop whining, and go try to make his case to get votes."
Debate prep
Obama has campaigned on several occasions for the 68-year-old Clinton, who served as his first secretary of state and who is riding high in the latest national and battleground state polls.
Clinton is almost certainly happy to try to ride out the Trump turmoil while pushing her policy agenda.
The third and final presidential debate will be a 90-minute war of words Wednesday at the University of Nevada in Las Vegas, beginning at 6:00 pm (0100 GMT Thursday).
The program is expected to address the issues of the US debt, immigration policy, the economy, the Supreme Court, hot spots around the world, and the candidates' preparedness to be commander in chief.
As usual, Clinton has carefully prepared, and had no public events scheduled for Monday and Tuesday.
When asked how debate preparation was going by a reporter at the Westchester airport, the Democrat gave a thumbs up sign as she boarded her plane to Las Vegas.
Trump on Tuesday was in Colorado Springs, engaging in what one of his aides described as "debate prep" in the morning.
But he refuses to slow down on the campaign trail. He heads to a 1:00 pm (1900 GMT) rally in the city, and then holds a second rally in Colorado, in Grand Junction, before heading to Vegas.
WATCH: Trump claims election is 'rigged'
Voter fraud 'very common'
Trump has sought to fire up his supporters, who have grown more aggressive by the day towards his Democratic opponent, as they and Trump fume over Clinton's swirling email scandal, and argue that the election is rigged in her favor.
"People that have died 10 years ago are still voting, illegal immigrants are voting," Trump claimed at a rowdy rally Monday in Green Bay, Wisconsin.
"Voter fraud is very, very common."
Obama dismissed Trump's accusations about rigged elections as "based on no facts."
Experts and Republican Party officials have also denounced the 70-year-old Trump for his accusations.
But in the particularly destructive climate of US politics in 2016, 41 percent of American voters, including 73 percent of Republicans, believe the election could actually be stolen from Trump, according to a poll by Politico and Morning Consult.
Melania defends Trump
Trump's campaign has reeled in the face of his lewd comments about women and accusations of sexual assault from several women.
His team has deployed his wife Melania in a media blitz to try to tamp down the furor over the allegations, with interviews airing late Monday on CNN and early Tuesday on Fox News.
"Those words, they were offensive to me and they were inappropriate. And he apologized to me. And I accept his apology. And we are moving on," Trump told Fox.
A firestorm erupted earlier this month when a 2005 video was made public and caught Trump saying lewd things about women, in a mostly off-camera conversation with host Billy Bush of the show "Access Hollywood."
Melania Trump told CNN that she felt her husband had been "egged on by the host to say dirty and bad stuff."
Bush has since been fired by NBC from his position on the "Today" show.
Trump declared his innocence once again Monday. Asked on ABC whether he had ever crossed the line with women, Trump said "I don't think so. I have great respect for women."
"It's lies, pure lies," he said of the accusations, even suggesting the women might be "part of the Clinton campaign."
Riding high in the latest national and battleground state polls, Clinton is looking to expand the electoral map, investing in states like Arizona and Missouri that are traditionally Republican-leaning states.
