What exactly is the Medicare guarantee?

As Paul Ryan revs up Republican engines for Medicare privatization, Nancy Pelosi vows to Greg Sargent that Democrats will fight it to the death:

Pelosi adamantly stated that Democrats would not give any ground on the core ideological dispute here, which is over whether to maintain a government coverage guarantee. “We are not going to a casino — this is a guarantee,” Pelosi said. “This is a value system for us, and we will fight for it. Is it a guarantee, or not?”

This raises a question: what precisely is the Medicare guarantee?

At present, there's a pretty specific answer: for 95% of seniors, the federal government will pay about 85% of the premiums for insurance that covers a bit more than 80% of the average user's medical costs. That's what traditional Medicare does right now, via Parts A, B and D, for those whose incomes are below $85,000 for a single person or $170,000 for a couple.

Put another way, the federal government pays a bit more than two thirds of the average senior's total medical costs. Low income beneficiaries have all or part of their premiums and out-of-pocket costs paid by Medicaid, though a variety of programs. High income seniors pay higher shares of their premiums, with the percentage stepped up through several income brackets.

One big happy future family: Romneycare, Obamacare, Ryancare

On a second read of the Medicare reform plan outlined in Paul Ryan's 2013 budget, I was whipsawed by crosscurrents of irony.

I was going to lambaste the plan for its touching faith in the Competition Fairy, believed to shower her low cost/high quality beneficence on private insurance plans competing in a government-run and -financed marketplace. Then it occurred to me that the ode I was reading was virtually identical to the praises sung by Democrats for the prospective healthcare exchanges to be established by the Affordable Care Act. Indeed, the ode may have been composed in large part by a Democrat -- Ron Wyden, who coauthored a proposal with Ryan in December 2011 that Ryan's more recent plan resembles more than it departs from (notwithstanding important differences).  Here's part of the opening movement  of Ryan's Song -- emphasis (and repetition) in the original: