In the first real effort to organize Senate Democrats behind a demand that Obama stand firm against GOP efforts to do away with Medicare as we know it, Sherrod Brown is circulating a letter among Senate Dems urging Obama to give the thumbs down to any proposal that would alter the program’s basic mission.

The letter — which was sent over by a source — comes as some Democrats are imaginging the worst: That the President will not make it clear in his speech responding to Paul Ryan tomorrow that any proposals that tamper with the core idea underlying Medicare are a total non-starter. If Brown’s letter garners a lot of signatures within the Dem caucus, which it is likely to do, it could serve to draw a firm line.

The letter denounces the Ryan proposal as morally wrong for seeking to undo the social safety net, accuses Republicans of shifting the burden of deficit reduction downwards, and reminds the president of Medicare’s historic success in lifting seniors out of poverty:

We are writing to urge you to oppose turning Medicare into a voucher system as proposed in the House Republican’s FY2012 budget. Since the enactment of Medicare in 1965, America’s seniors have no longer lived in fear of losing affordable, comprehensive health insurance when they retire. Unfortunately, some in Congress want to dismantle Medicare in order to help offset the costs of tax cuts for the very wealthiest in our country.

The House Republican budget for Fiscal Year 2012 would end Medicare as we know it and throw seniors into the private market with no more than an insufficient voucher to offset the rising cost of private health insurance. So-called “premium support” — giving seniors a voucher of approximately $8,000 as proposed by the Republican budget — is a reckless and irresponsible way to address the health care needs of older Americans. And it is an unacceptable means by which to finance tax cuts for those who are earning ten times or more than the retirement income of the average Medicare recipient. Seniors, who have paid into the system their entire working lives, deserve affordable, secure health coverage upon retirement...

Before the passage of Medicare, only half of America’s seniors had health insurance, and most of those with insurance only had coverage for inpatient hospital costs. Additionally, approximately 30 percent of seniors lived below the poverty line before Medicare. Now, only 1.8 percent lack health coverage and less than 9 percent live below the poverty line. We cannot afford to reverse these gains through the ultimate form of rationing health care for seniors: the replacement of Medicare as we know it with insufficient vouchers for private health coverage. We urge you to protect America’s seniors and oppose any attempts to dismantle Medicare.

The letter, interestingly, is directed at the President, essentially demanding that he reaffirm his commitment to Medicare as we know it or be seen failing to protect “America’s seniors.” It’s a throwing down of the gauntlet of sorts that will hopefully garner significant support among Senate Demcrats, and suggests liberal Senators will not accept anything but a flat out rejection of the Ryan plan as a total conversation-ender.