Senator Mitt Romney, of Utah, offered more than 200 Republican donors a stark message on the fragility of American democracy during private remarks on Monday night at a fundraiser in Northern Virginia.

According to five attendees, Romney told the crowd that he has a chart in his Senate office tracing the history of civilizations over the past 4,000 years. He said it is a reminder of how they can rise and collapse, and of how unusual American democracy is in global history.

From the Mongol Empire to the Roman Empire, Romney said, autocracy is the chart’s “default setting,” with authoritarian leaders at every turn.

“We are really the only significant experiment in democracy, and preserving liberal democracy is an extraordinary challenge,” Romney said, according to the attendees, who gathered at the Hilton Hotel in McLean.