Margot Sanger-Katz, a National Journal reporter who has been brave enough to question conventional wisdom surrounding health policy—she reported that elements of the Affordable Care Act “designed to lower costs will likely raise them instead”—has now taken a hard look at the claims and rhetoric sloshing around about vouchers lowering the government’s Medicare bill.

She asked a reasonable question, one the media haven’t examined much: Will vouchers, which will likely result in seniors and disabled people paying more for their care out of pocket, really save money for Medicare, as some politicians, mostly Republicans and notably Mitt Romney and Paul Ryan, have claimed?

According to the theory, Medicare beneficiaries would be able to take their government subsidy, or voucher, and venture into the private insurance market. Sellers would then compete for their business and thus, in theory, lower the price of insurance and create downward pressure on medical costs.

How can the press report on this theory? Instead of relying on a tiresome and not very useful he said/she said formula—the Dems say vouchers won’t work; the GOP says they will—Sanger-Katz dug in and asked, “Where’s the beef?” when it comes to vouchers. Here’s what she found:

The logical-sounding notion of vouchers bringing more competition and thus lower prices is

so untested that even the experts who are the most enthusiastic about the approach say they don’t know to what degree competition will slow spending. No one this cycle—not the Congressional Budget Office, not the Heritage Foundation, not even the Romney campaign—has estimated the effect. Romney’s plan could generate savings, but even proponents admit that it would amount to a huge gamble on a yet-unmeasured mechanism for slowing growth.

Sanger-Katz checked in with some voucher advocates. One academic booster she interviewed was Roger Feldman, a professor at the University of Minnesota, who has written a lot about vouchers. In a recent paper for the conservative American Enterprise Institute, a think tank that has long been a voucher enthusiast, Feldman estimated that Medicare could save more than $339 million over 10 years, most of it coming from a few large markets where the government apparently overpays for care.

But here’s the problem: In a few markets, it seems, private plans can provide care cheaper than Medicare. But after that, there’s no evidence that what it costs Medicare to pay for care for beneficiaries is affected. In other words, there is no evidence that vouchers slow the growth rate for Medicare spending, which is driven more by doctors’ and hospitals’ bills than by the insurers who pay them. When it comes to curbing the growth of Medicare spending, Singer-Katz reports, “Feldman makes no promises.” He told her, “I don’t build that in because I don’t think the evidence is strong enough.”

Trudy Lieberman is a fellow at the Center for Advancing Health and a longtime contributing editor to the Columbia Journalism Review. She is the lead writer for The Second Opinion, CJR’s healthcare desk, which is part of our United States Project on the coverage of politics and policy. Follow her on Twitter @Trudy_Lieberman.