The withering riposte, in language usually used to scold a moody teenager, came on the eve of the third and final presidential debate between Trump and Democrat Hillary Clinton.
Obama discarded diplomatic decorum, skewering the Republican mogul from the Rose Garden in front of visiting Italian Prime Minister Matteo Renzi.
Trump has ramped up conspiracies about America’s election system as his poll numbers have plummeted in the wake of sexual assault allegations against him.
He trails Clinton by around seven points nationwide and bookmakers in Europe where political betting is legal have already begun to pay out on a Clinton win.
But the White House is increasingly concerned that Trump and his supporters will not recognize the election’s outcome, plunging the country into a political crisis.
According to a poll by Politico and Morning Consult, 41 percent of American voters, including 73 percent of Republicans, now believe the vote could actually be stolen from Trump.
“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 said.
“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.
Addressing Trump’s allegations of “large scale voter fraud,” Obama said, “There’s no evidence that that has happened in the past, or that there are instances in which that will happen this time.”
“I’d advise Mr Trump to stop whining and go try to make his case to get votes.”
Trump and Clinton jetted in separately Tuesday to Las Vegas, the entertainment capital in the Nevada desert, ahead of their final debate Wednesday.
With three weeks until the November 8 election, it may be Trump’s last chance to make a positive mark on millions of voters.
Campaigning Tuesday in Colorado, a slightly subdued Trump indicated his scorched-earth tactics would continue.
“We’ve only just begun to fight, believe me,” he said. “This is our final shot, folks,” he told a rally in Grand Junction.
Polls giving Clinton the lead were inaccurate, he insisted. In fact, “they’re sort of good” for the Trump campaign.
“We are going to have one of the greatest victories in political history,” he predicted, excoriating the US media for being “an extension of the Hillary Clinton campaign.”
“The press has created a rigged system and poisoned the minds of the voters,” he said.
“Either we win this election or we lose the country.”
Trump doubled-down on his vote rigging claims, saying “voter fraud is all too common.”
His dark call to monitor polling stations in cities with large populations of African Americans, who overwhelmingly favor Clinton over Trump, could be interpreted as pre-election intimidation.
“If nothing else, people are going to be watching on November 8,” he told Colorado supporters.
“Watch Philadelphia. Watch St. Louis. Watch Chicago.”
Experts and Republican elected officials have denounced Trump for accusations of voter fraud, prompting the nominee to turn on his own party.
He went a step further Tuesday, calling for term limits for every member of Congress: 12 years in the US Senate and six in the House of Representatives.
But Clinton’s campaign believes Trump’s hot rhetoric has helped Democrats — not only by galvanizing supporters but also shifting undecided voters to their camp.
“We know that he thought that strategy of scorched earth would depress our vote, but if anything, we have found that it’s helped to motivate our voters,” Clinton communication’s director Jennifer Palmieri said.
Democrats are beginning to target traditionally Republican states in a bid to run up the score and help win legislative races that could decide who controls Congress.
On Thursday, First Lady Michelle Obama will campaign in Arizona, which has not voted Democratic since Bill Clinton’s landslide win against Bob Dole in 1996.
Tom Lazzaro, a 45-year-old who owns a real estate company in Colorado, said he worried about electoral fraud.
“If there’s a lot of red states that suddenly turn blue I’m going to really question that,” he told AFP at Trump’s rally in Colorado Springs.
But retiree Mike Bergst, a Trump supporter, said the nominee’s responsibility was to defeat Clinton fair and square.
“He needs to win the election, bottom line, whether it’s rigged or not,” said 60-year-old Bergst.
“If he loses, I’m not going to go around saying it was fixed.” -AFP