Billionaire investor Howard Marks said he's worried about an increasing sentiment taking hold in American politics.

"There is a rising tide of anti-capitalism and we should be concerned about that," the co-chairman of Oaktree Capital said Thursday on CNBC's "Fast Money: Halftime Report."

"We have a machine in this country that makes it successful — based on democracy, our freedoms and also I think the economy and the way it operates in a free-market mode."

Marks addressed the issue in his latest memo to clients. He's known for his prophetic notes, which have warned about the 2008 financial crisis and the dot-com bubble of the late 1990s. He's now predicting the far-left wing of the Democratic Party will "show up strongly" in the 2020 primaries.

He called out the "increasing anti-capitalist sentiment" and specifically took issue with New York Rep. Alexandria Ocasio-Cortez's proposal targeting the wealthy with a 70 percent marginal tax rate on income above $10 million.

Marks also criticized the proposed wealth tax from possible 2020 presidential candidate Sen. Elizabeth Warren. The Massachusetts Democrat wants to impose a 2 percent tax every year on households with assets over $50 million. That rate would rise to 3 percent on households with assets over $1 billion.

"Americans generally accept the concept of progressive tax rates. But they must not be punitive and demotivating," he wrote.

On CNBC, he said he understands that income inequality has expanded and that something should be done about it. "The cost of capitalism is that there are some people who succeed a lot and some people who don't succeed. And that's undesirable," he said. "We should have a safety net for them. But I still think we shouldn't throw out the system."

Marks also blames the Federal Reserve for exacerbating the wealth gap, but said the central bank did what it needed to do during the 2008 financial crisis. At the time, the Fed slashed interest rates to near zero and took other measures to stabilize the economy. "A side effect of that was asset inflation, which of course only benefited the people who owned assets and that has created a widening of the net worth gulf."

Oaktree Capital had $124 billion of assets under management as of September. Marks has a net worth of $2 billion, according to Forbes.