The ten liberal and moderate Democrats trying to hammer out a compromise on the public option are talking seriously about letting older workers pay to be enroll in Medicare. While doing so wouldn't qualify as creating a new public plan into which anybody could enroll, it would qualify as opening up an existing public plan to a group of people that might appreciate its many virtues. And that's certainly a good thing.

Introducing new ideas at this relatively late stage of the debate is not exactly easy to do. But, then, this isn't exactly a new idea. President Clinton proposed a version of it in the late 1990s. He saw it as a stand-alone step, something that could help expand coverage incrementally. More recently, the Commonwealth Fund proposed a Medicare buy-in as part of its "building blocks" proposal, which closely resembles the schemes moving through Congress now. And just this spring, Senators Jay Rockefeller (himself a Gang of Ten member) and Ben Cardin introduced the "Medicare Early Access Act."

How would such a plan work in practice? That really depends on the parameters--which, I gather, are very much up for negotiation right now. Among the key questions:

Who gets to enroll? Most of the past proposals limited Medicare enrollment to people without access to employer coverage or existing government programs. Some would open up Medicare to any such person over 55. Others restricted enrollment to only those who met certain criteria (they had a disability, had recently lost a job that offered coverage, etc.)

When do people enroll? One possibility would be to delay this program until the exchanges come on line, in 2014. Another would be to open Medicare enrollment earlier. That would provide more immediate help to a group that very much needs it. (Older working Americans without coverage have a predictably tough time finding affordable coverage on the open market, because their age makes them higher health risks.) On the other hand, opening up the program early could cost more money, although that depends in good part on...

What will people pay? Typically in these schemes, the existing Medicare program for seniors remains financially separate. Early workers can buy the coverage, but their dollars remain separate. In fact, under every proposal I've seen, the idea is to make the program financially sound, by charging workers premiums that reflect their expected medical expenses. Exactly how that translates, on a per person basis, depends in part on whether Medicare enrollment begins before the exchanges open up--and whether, either way, there are subsidies for people who can't pay the premiums on their own.

For those more interested in the policy background, here's a paper on the subject from the Kaiser Family Foundation.