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President Barack Obama on Thursday delivered his most stinging rebuke yet of Donald Trump’s rigged election rhetoric, calling it “dangerous” and saying the Republican nominee is “doing the work of our adversaries.”

“When you suggest rigging or fraud without a shred of evidence, when last night at the debate Trump becomes the first major party nominee in American history to suggest that he will not conceded despite losing the vote, and then says today that he will accept the results if he wins, that is not a joking matter,” Obama said during a campaign stop for Hillary Clinton in Miami Gardens, Florida.

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Trump refused to say he would accept the results of the election during the final presidential debate on Wednesday. He followed that by holding a rally Thursday in which he proclaimed he would accept the outcome on November 8, “If I win.”

He later added that he would accept a “clear” election result.

“That is dangerous because when you try to sow the seeds of doubt in people’s minds about the legitimacy of our elections, that undermines our democracy,” Obama continued. “Then you’re doing the work of our adversaries for them. Because our democracy depends on people knowing that their vote matters.”

As recently as Tuesday, the president went after Trump for his continued talk about a “rigged election.”

Obama advised Trump to “stop whining,” adding that the GOP’s complaints show he’s not ready to be president.

The Republican has yet to offer any evidence the U.S. electoral process has been compromised. At the third debate he said media bias has “poisoned the minds of the voters” and Clinton’s scandals should have prevented her from running.