The Wyden-Ryan plan — still lacking cost projections and other key details — is unlikely to go anywhere before the 2012 elections, but it could serve as a marker of sorts in the political debate next year.

Medicare covers 47 million Americans, and the oldest baby boomers started signing up for benefits this year. But the program's long-term financial outlook is shaky. Cuts to service providers, cost shifts to Medicare recipients and reduced benefits for future retirees were all options on the table in this year's failed budget talks between Congress and the White House. Tax increases can't be ruled out, either.

Under the Wyden-Ryan plan, current beneficiaries and those close to retirement would get to remain in Medicare as it is now.

But the program would be re-engineered for those 54 and younger. Upon reaching 65, future retirees would have a choice between traditional Medicare and regulated private insurance plans, all competing to lower costs and provide quality care. Seniors would get a fixed amount to spend on a health plan, no matter which coverage they selected. Low-income, and older, sicker people would get more money.

The plan also would limit the overall increase in Medicare spending, holding it to no more than 1 percent above the rate of economic growth.

The White House reacted sharply. "We are concerned that Wyden-Ryan … would undermine rather than strengthen Medicare," said spokesman Jay Carney.

"At the end of the day, this plan would end Medicare as we know it for millions of seniors," he added.

Earlier, communications director Dan Pfeiffer said in a statement the Wyden-Ryan plan would cause Medicare to "wither on the vine." Carney later repeated that same phrase at the White House news briefing for reporters.

But a spokeswoman for Wyden said the senator's plan is not a radical departure from previous health care proposals made by Democrats, including President Barack Obama. An early version of Obama's health care overhaul also called for a competition between private insurance plans and a so-called "public option," said spokeswoman Jennifer Hoelzer. In that case however, the issue was health insurance for workers and their families, not seniors.

"Sen. Wyden and Rep. Ryan are attempting to engage Democrats and Republicans in a constructive conversation about how to best save and strengthen Medicare," said Hoelzer. "It would be more productive if others would join in that conversation instead of attacking it."

Wyden got his start in politics as an advocate for seniors. Ryan is best known as the architect of this year's House GOP budget, which included a proposal to essentially privatize Medicare for future retirees. That plan triggered a political backlash and was rejected by the Democratic-led Senate, as well as Obama.

The new Wyden-Ryan plan closely resembles a proposal recently released by Romney, who calls it a "premium support" approach for Medicare. Seniors would get taxpayer support to pay their health insurance premiums, but that subsidy would have to have certain limits. Like Wyden and Ryan, Romney preserves traditional Medicare as an option alongside private insurance plans.

"We are very pleased to see that congressman Ryan has introduced a Medicare reform proposal that aligns so closely with what Gov. Romney proposed last month," said spokeswoman Andrea Saul.

Late Thursday, AARP weighed in with a cautious if skeptical reaction. Nancy LeaMond, vice president of the seniors' lobby, said the group will withhold judgment for now because of the lack of details about the plan. However, "even at this high level, obvious risks come to mind," she added.