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 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."
SBS Senior Correspondent Brett Mason is in the United States in the lead up to November's presidential election. He's been meeting the locals and getting a feel for how the vote might go on November 9.
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.
