The president said Donald Trump’s attempt to discredit a poll before it has even taken place was “unprecedented” for a presidential candidate.
Also “unprecedented”, said Barack Obama, was Donald Trump’s “flattery” of Russian President Vladimir Putin.
Donald Trump is facing sinking poll numbers and accusations of assault.
The New York billionaire has claimed the next month’s election will be “absolutely rigged” for Democratic candidate Hillary Clinton.
In a White House Rose Garden news conference on October 18 alongside visiting Italian PM Matteo Renzi, President Obama said Donald Trump’s assertions were “based on no facts”.
“I’d advise Mr. Trump to stop whining and try to make his case to get votes,” Barack Obama said.
“By the way,[it] doesn’t really show the kind of leadership and toughness that you want out of a president, 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.”
President Obama also addressed Donald Trump’s admiring remarks about Vladimir Putin.
“Mr. Trump’s continued flattery of Mr. Putin and the degree to which he appears to model much of his policies and approach to politics on Mr. Putin is unprecedented,” Barack Obama said.
The president’s broadside comes a day after Donald Trump said he would consider visiting Russia before taking office, if elected.
Donald Trump told a talk-radio host: “If I win on November 8, I could see myself meeting with Putin and meeting with Russia prior to the start of the administration.”
Hillary Clinton and Donald Trump face off in the third and final presidential debate at the University of Nevada, Las Vegas, on October 19.
The Republican nominee has seen his poll numbers slide since their first battle of wits, followed by the emergence of the 2005 tape that he was forced to address in the second debate.
Donald Trump denied any impropriety, but a number of women have come forward with assault allegations against him.
Hillary Clinton will head into October 19 debate with a seven-point lead over Donald Trump, according to a Monmouth University poll. Hillary Clinton leads Donald Trump 47% to 40%, while 7% of likely voters say they will support the Libertarian Party candidate, Gary Johnson.
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