Two more Democratic senators are introducing a bill that would create a version of Medicare for some working-age Americans, offering yet another sign that government-run insurance will figure prominently into the Democratic Party s health care agenda going forward.

Sens. Jeff Merkley (Ore.) and Chris Murphy (Conn.) plan on Wednesday to introduce the Choose Medicare Act, which would launch a new, enhanced version of traditional Medicare. It would be available both to individual consumers buying their own coverage and employers who offer coverage to employees.

Its possible to imagine Medicare Part E, as Merkley and Murphy propose to call the new program, someday absorbing most working-age people who dont qualify for Medicaid. If that happened, the American health care system would more closely resemble the kind of single-payer system that exists in some countries abroad and many progressives would prefer to have in the United States.

Of course, whether Medicare Part E would really prove as attractive as Merkley and Murphy say and precisely what trade-offs the full legislation would entail remain very much open questions.

Medicare Part E would include two features of commercial insurance that traditional Medicare, which serves seniors, does not: a limit on out-of-pocket spending and coverage of pediatric services.

It would have to finance itself, with premiums from beneficiaries covering outlays, just like a private insurance plan would. Medicare Part E would benefit from some of Medicares efficiencies among other things, by paying less for services than commercial insurance typically does. Merkley and Murphy are counting on that differential to make Medicare Part E competitive on price and coverage.

Consumers who buy coverage on their own could get Medicare Part E through HealthCare.gov or one of the state-run exchanges, using federal tax credits if they qualify. The tax credits would actually be more generous, and available to people at higher incomes, than they are under the current law.

The legislation wouldnt only help people who opt into Medicare E. People who buy coverage on the exchanges but prefer to stick with private insurance would also be eligible for the bigger, more widely available tax credits.

Seniors would also get something. The Merkley-Murphy bill gives traditional Medicare a cap on out-of-pocket expenses, something it currently lacks.

Bolstering the subsidies and traditional Medicare would require additional funds. And if history is any guide, making all of these numbers add up would prove difficult once the legislation is subject to more detailed analysis. As details of the plan became more clear, it would likely generate political opposition.