U.S. Sen. Bernie Sanders warned late Monday that billionaire Elon Musk’s new call for up to $700 billion in cuts to mandatory federal spending is an alarming step in the direction of Social Security privatization, a longstanding—and deeply unpopular—goal of right-wing politicians and corporate-funded think tanks.

Musk, who is spearheading a large-scale assault on federal agencies and workers, told Fox Business host Larry Kudlow on Monday that “waste and fraud” in “entitlement spending”—a category that includes Social Security, Medicare, and Medicaid—is “the big one to eliminate,” estimating that up to $700 billion could be cut from such programs.