EARLIER this month, when the billionaire merchandising mogul Johann Rupert gave a speech at The Financial Times’s “luxury summit” in Monaco, he sounded more like a Marxist theoretician than someone who made his fortune selling Cartier diamonds and Montblanc pens. Appearing before a crowd of executives from Fendi and Ferrari, Mr. Rupert argued that it wasn’t right — or even good business — for “the 0.1 percent of the 0.1 percent” to raid the world’s spoils. “It’s unfair and it is not sustainable,” he said.

For several years now, populist politicians and liberal intellectuals have been inveighing against income inequality, an issue that is gaining traction among the broader body politic, as shown by a recent New York Times/CBS News poll that found that nearly 60 percent of American voters want their government to do more to reduce the gap between the rich and the poor. But in the last several months, this topic has been taken up by a different and unlikely group of advocates: a small but vocal band of billionaires.

In March, for instance, Paul Tudor Jones II, the private equity investor, gave a TED talk in which he proclaimed that the divide between the top 1 percent in the United States and the remainder of the country “cannot and will not persist.” Mr. Jones, who is thought to be worth nearly $5 billion, added that such divides have historically been resolved in one of three ways: taxes, wars or revolution.

A few months earlier, Jeff Greene, a billionaire real estate entrepreneur, suggested on CNBC that the superrich should pay higher taxes in order to restore what he called “the inclusive economy that I grew up in.”