Paul Ryan is in Florida today promoting his Medicare plan, but there's a part of his plan that he very carefully avoided talking about: what he'll do if his cost controls don't work.

If you really want to hold down costs, you have to hold down costs at the source, and Paul Ryan's Medicare plan has no mechanisms for doing this. It relies solely on competitive bidding, and there's very little chance that this alone can keep Medicare costs from outpacing his "fallback" growth cap. It's a near certainty that his growth cap will be the real mechanism for reining in costs.

So again the question is: what happens if health providers bid for Medicare contracts but the bids all come in higher than Ryan's growth cap? Do premiums go up for seniors? Ryan doesn't say so, but then again, he also refuses to say what would happen. But there's no fairy dust here. If costs under the Ryan plan go up more quickly than his growth cap — and they almost certainly will — then someone has to pay the difference. And that someone is either beneficiaries or taxpayers or healthcare providers. There aren't any other choices.

But it can't be taxpayers, because that undermines the whole point of the plan. And it can't be providers, because Ryan's plan has no mechanism for cutting payments to providers. In fact, he and Mitt Romney have recently abandoned even the existing provider cuts contained in Obamacare. So all that's left is seniors. Every year their voucher will cover less and less of the cost of the cheapest plan, and seniors will have to pay more and more out of their own pocket. By refusing to address this issue, Ryan has successfully kept things vague enough that no one can produce hard numbers about how much more seniors would end up paying. But it would be a lot.