Supporters for the Romney-Ryan approach to Medicare have a new talking point. They say a new study by “three liberal Harvard economists” proves that the plan’s competition will reduce health care costs without harming beneficiaries. But the study doesn’t say that.

And I should know. I’m one of the economists who wrote it.

Both Mitt Romney and Paul Ryan have said they would like to convert Medicare into a "premium support" (nee voucher) system. Their plans are different, and Ryan himself has proposed several versions. But they share a basic architecture. Starting ten years from now, new retirees would not receive a Medicare card, as they would today. Instead, they would receive a voucher and shop for an insurance policy in a specially regulated market.

The voucher would equal the price of the second-cheapest plan in the market, although its value would be less if insurance prices rose faster than a pre-determined spending cap (of gross domestic product plus half a percentage point)—as they are projected to do. Both Romney and Ryan now say that traditional Medicare, the government-run insurance program, would be among the options in the marketplace. But they would not guarantee that voucher can pay for it. In fact, that’s very much the point of the proposal: To create more competition between Medicare and private plans, even if that means Medicare ends up costing more than the vouchers are worth.

How would this affect seniors? In particular, how many seniors would end up paying more to stay in traditional Medicare?