Democratic senators led by Elizabeth Warren urged top Biden health officials to take a harder line on private Medicare Advantage health plans, arguing that insurance companies annually overcharge the government by tens of billions of dollars.

The lawmakers asked the administration to ensure “funds that are incorrectly going to” Medicare Advantage firms go instead to patients and medical providers, according to a letter sent late Monday to the leaders of the Department of Health and Human Services and Centers for Medicare and Medicaid Services.

The Biden administration has taken “unprecedented steps to improve transparency, use taxpayer dollars wisely, promote competition, and make sure people can get the care they need quicker” through Medicare Advantage, while cracking down on abusive marketing tactics, an HHS spokesperson said in an email. CMS didn’t respond to a request for comment.

Medicare Advantage, a private version of the federal health program for seniors, has driven growth and profits across the health insurance industry for years. The private plans covered 31 million people last year and the largest sellers — units of UnitedHealth Group Inc., Humana Inc. and CVS Health Corp. — enroll about half the market.

The business is facing fresh challenges, though, with a rise in medical care expenses colliding with federal efforts to tighten some payments. The senators’ letter highlights increasing concerns about how much the private plans cost taxpayers.

Overpayments to plans run as high as $156 billion a year, the senators wrote, citing an analysis by the Committee for a Responsible Federal Budget. Congressional advisers at the Medicare Payment Advisory Commission estimate the health program will pay $88 billion more for Medicare Advantage this year than it would cost to cover the same people in the traditional version of the program. For years, the nonpartisan group has called for revamping Medicare Advantage payments.

The Massachusetts senator and her colleagues asked CMS to speed up changes that would “cut down on insurers’ misuse of medical codes to increase payments,” a practice called upcoding. They also asked for other technical changes that would result in lower payments to Medicare Advantage plans.

Other senators signing the letter include Richard Blumenthal of Connecticut, Cory Booker of New Jersey, Sherrod Brown of Ohio, Richard Durbin of Illinois, Edward Markey of Massachusetts, Jeff Merkley of Oregon, Tina Smith of Minnesota, and Vermont’s Peter Welch and Bernie Sanders, an independent.

The senators urged agency leaders to scrutinize industry practices that limit patients’ access to care — including the use of artificial intelligence in medical decisions. They also pointed to longstanding concerns about potentially misleading marketing for the private programs and transparency about costs to patients.