Story highlights Donald Trump and his surrogates have recently claimed that the US election system is 'rigged'

Obama derided Trump's remarks as reflective of an unpresidential attitude

Washington (CNN) President Barack Obama on Tuesday cast Donald Trump's claims of a rigged election as potentially corrosive to American democracy, insisting that the Republican presidential nominee was griping about an invented conspiracy.

"You start whining before the game's even over?" Obama said during a news conference in the White House Rose Garden, adding that Trump's claim is "not based on facts."

Trump and his surrogates have increasingly claimed the US election system is "rigged," coming after two lackluster debate performances and a drop in poll numbers nationally and in key swing states. He's urged his supporters to monitor polling sites for potentially ineligible voters attempting to cast ballots.

The rhetoric has been brushed off even by Republican governors, who say there are no signs of corruption in their states' voting systems.

Obama echoed those sentiments Tuesday, saying there's "no serious person out there who would suggest somehow that you could even rig America's elections."

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