Salon's Andrew Leonard has a fascinating history of the politics of Medicare, beginning with this ad the Johnson campaign ran against Barry Goldwater, a month before the 1964 election. Goldwater interrupted his campaign to return to DC "for the sole purpose of voting no on a key precursor to what became Medicare."
Democrats already controlled the House and Senate before the 1964 election, but a coalition of Republicans and conservative Southern Democrats had hitherto managed to stymie every attempt to make that crucial first step toward what the American Medical Association's spokesperson Ronald Reagan decried as "socialized medicine." But on Election Day, Johnson's landslide victory ushered in majorities so huge that reaching a goal Democrats had been seeking since Harry Truman's first term finally became inevitable. Johnson's mandate was undeniable, and even the powerful Arkansas Democrat Wilbur Mills, chairman of the all-important House Ways and Means Committee, who had long stalled a House vote on Medicare, was forced to acknowledge it. As Johnson told Vice President Humphrey in March 1965, with the kind of Southern slang laxative analogy that only he could pull off, in the new political atmosphere, "Medical care will go through like a dose of salt through a widow woman." And he was right....But from the vantage point of 2011, the question of whether 1964's perfect storm permanently altered the healthcare landscape of the United States now seems open to question. Medicare is still hugely popular, but a combination of rising costs (driven by demographic changes and accelerating medical expense inflation) and decades of Republican success at reducing the relative proportion of taxes paid by Americans, has resulted in a Catch-22. We like our expensive social welfare system, but we aren't willing to pay for it. Even worse, as it gets more expensive, we are willing to pay less and less! Furthermore, the Republican Party has never accepted its defeat in 1964—in fact, you can easily argue that the rise of the modern conservative movement got its start from the twin motivations of Goldwater's debacle and Johnson's Great Society successes.
The budget proposal released last week by House Budget Committee Chairman Paul Ryan includes the most substantial assault on Medicare and Medicaid as we know them in decades. And even if Ryan's agenda breaks on the rock of popular outcry and Senate Democratic resistance, that still doesn't solve the problem of how to pay for the programs. Obama, far from being remembered as the president who expanded healthcare coverage for all Americans, might end up as the leader who presided over the first significant retrenchment of these pillars of the Great Society.
Which makes it all the more amazing to go back and look at the legislative sausage making involved in the creation of Medicare and Medicaid. Because if there's anything that jumps out from accounts of the struggle Republicans waged against any involvement by the government in medical care, it was how similar their proposals were to important aspects of the Affordable Care Act today. Republicans attempted to fend off the specter of socialism by proposing plans in which the government would provide subsidies to private insurers that supposedly create incentives encouraging them to cover the elderly. Sound familiar?
....Today, once again, just about everybody recognizes that we face a serious problem. The ranks of the uninsured are growing, medical cost inflation is out of control, and something needs to be done. But once again, Republicans still call attempts to involve the government in a solution "socialized medicine." Only this time, what they call socialism is darn close to what they used to support as an alternative to the heavy hand of government.
It's not your father's Republican party, anymore. But Medicare has only grown in popularity, even among Republicans:

With that reality, the kind of evisceration of Medicare that Ryan lays out is pretty unlikely to happen. You can bet that most rank and file Republicans are counting on Obama and the Senate Dems to make sure of that, because they don't want to have to run for re-election on the "we killed Medicare" platform. But that should by no means make any supporter of the program sanguine. The Republicans' fight against the New Deal and Great Society have been unceasing. As of 2011, they have achieved moving the center of the debate over these programs so far to the right that, for the first time, they sense success in the near future. Stopping them now is critical, but moving the debate back to sane ground is paramount.