US President Barack Obama warned Friday that American "democracy itself" is on the ballot in November's presidential election, as White House concern grows about the lasting impact of Republican Donald Trump's scorched-earth campaign.

At a fiery campaign event for Hillary Clinton in Cleveland, Ohio, Obama trashed Trump as a dictator-in-the-making, but also voiced concerns about how Trump's legion of supporters might react to a possible election defeat.

Trump has in the last week declared himself free from the shackles of normal political etiquette and hurled a series of highly inflammatory accusations against Clinton and her husband.

"Civility is on the ballot" on November 8, Obama told a group of largely young voters in the swing state of Ohio.

"Tolerance is on the ballot," he continued. "Courtesy is on the ballot. Honesty is on the ballot. Equality is on the ballot. Kindness is on the ballot. All the progress we made in the last eight years is on the ballot," he said. "Democracy itself is on the ballot right now."

As Trump has tanked in polls, his campaign in chaos over a damning video tape and allegations of sexual impropriety, thoughts have turned to whether the controversial real estate mogul would even acknowledge defeat should he lose.

He has spent the last week claiming the media and a "global elite" is working against him.

"Hillary Clinton meets in secret with international banks to plot the destruction of US sovereignty in order to enrich these global financial powers, her special interest friends and her donors," Trump told supporters in Florida.

Obama gave that claim short shrift. "C'mon man!" he said. "This is a guy who spent all his time hanging around trying to convince everybody he was a global elite."

"All he had time for was celebrities and now suddenly he's acting like he's a populist out there."

"This is somebody who... is now suggesting that if the election doesn't go his way, it's not because all the stuff he's said, but it's because it's rigged and it's a fraud," Obama said.

"He seems to be in the middle of the game making excuses all the time for why he might be losing," he said.

"You don't start complaining about the refs before the game's even done. You just play the game, right?"

- 'Crazy talk' -
With an eye on winning back control of Congress, Obama also used the rally in Cleveland to lash Republicans tight to their sinking White House nominee.

Clinton is now the favorite to win the presidency, but the balance of the Senate and the House of Representatives is much less clear.

"A lot of Republican elected officials have just stood by," Obama said. "They've allowed a lot of crazy talk to just be pumped out again and again."

"A lot of House members, a lot of Senators, they stood by and they didn't say anything because it was a way to rile up their base," Obama charged. "And that's what allowed Donald Trump suddenly to emerge."

That Trump-supporting base appears more fired-up than ever.

Obama on Friday became the latest in a wave of Democrats targeted by bounty-seeking agitators.

A few minutes into Obama's campaign riff selling his record over the last eight years in the White House, the outgoing 44th president noticed a commotion in the crowd.

"I notice this has been happening everywhere," Obama said.

Right wing website Infowars has put up a $5,000 reward for anyone who is heard and seen on television repeating the explosive, and unproven allegation that Bill Clinton is a rapist.

"If you're confident about the other guy, just go to his rallies. I feel confident about my candidate," Obama said.

On Thursday Infowars boasted in a headline: "'Bill Clinton Rape' movement explodes."