Cleveland, Ohio (CNN) Facing a seemingly endless presidential campaign, marked by a viciousness and vulgarity not seen in either of his own election contests, President Barack Obama has boiled his incredulity down to one phrase.

"Come on, man," a grinning Obama repeated twice during a lakeside campaign rally for Hillary Clinton here on Friday.

Deploying a sharp line against Trump's claims to be for the working class , Obama wondered why anyone would believe the nominee's claims.

"If a guy's spent 70 years on this earth showing no regard for working people, there's no record that he's supported (the) minimum wage or supported collective bargaining, invested in poor communities, and then suddenly he's going to be the champion of working people? Come on," Obama said.

He was speaking during his fourth campaign rally in support of Clinton, coming to Ohio as the window for early voting opens in this key electoral prize. Each of his stump speeches has sharpened his attack lines about the Republican Party's presidential nominee, from his avoidance of paying taxes to his treatment of women.

Drawing Obama's scorn Friday, Trump's insistence a "global elite" is working to rig the election

"This is a guy who spent all his time hanging around trying to convince everybody he was a global elite," Obama said. "Talking about how great his buildings are, how luxurious and how rich he is and flying around everywhere. All he had time for was celebrities and now suddenly he's acting like he's a populist out there. I'm going to fight for working people."

"Come on, man," Obama repeated.

Obama was speaking in Ohio a day after Trump claimed during a rally in Florida the entire US political system was corrupt.

"The only thing that can stop this corrupt machine is you," Trump said. "The only force strong enough to save our country is us. The only people brave enough to vote out this corrupt establishment is you, the American people."

It was the latest in a pattern of remarks from Trump casting doubts about the validity of the upcoming vote, claims Obama said have no place in US Democracy.

"This is somebody who threatens to jail his political opponents. Or silence the media, who welcomes Russian meddling in our electoral process, and 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.

And for the second day in a row, Obama hit back at Republicans who are now disavowing Trump, saying it's too little, too late. He spent Thursday night railing on lawmakers like Ohio's Sen. Rob Portman, who withdrew his support for Trump after his sexually aggressive remarks from 2005 were released on tape.

"I know that some of them now are walking away but why did it take you this long? You said you're the party of family values. What, you weren't appalled earlier when he was saying degrading things about women?" Obama said. "I'm glad that some of them now said 'wow, this is really bad, I guess we need to walk away,' but if you're doing it just for political expedience just because you're looking at poll numbers and you say 'Oh, this might get me in trouble,' that's not enough."