President Harry Truman, who tried to enact a national health care system, gives his 1949 inaugural address. Corbis via Getty Images

For years afterwards, the Democratic partys platform called for a "federally-financed and federally-administered...system of universal National Health Insurance," as the

But Democrats were thwarted by the large price tags, the policy complications, and the pernicious association with socialism, leading them to eventually conclude that only more modest reforms like Obamacare were possible. And support for the approach in the Senate among Democrats lags behind the House.

"There's a bit of a false dawn with single-payer that this is going to be popular even once details are known," said Jim Kessler, the senior vice president for policy at the centrist Democratic think tank Third Way.

"There's going to be tons of disruption," Kessler continued. "Maybe it's worth it, maybe it isn't. But before people sign on in a rush to it, we have to have a serious analysis of what it's going to mean for people and all the institutions involved."

"The ACAs changes to the health insurance system and the number of people affected by those changes has been small compared to the upheaval that would be brought about by the movement to a single-payer system," the Urban Institute noted in its

Indeed, the same polls that supporters cite to demonstrate the appeal of single payer also show that voters are responsive to negative arguments about costs and government control.

"While a slim majority favors the idea of a national health plan at the outset," wrote Liz Hamel and her colleagues at the nonpartisan Kaiser Family Foundation of their

Many Democrats worry their party is hurtling toward a policy commitment they dont fully understand when they should be focused on defending existing gains.

"We're one bad election away from the Affordable Care Act being repealed," said Kessler, referring to possible GOP gains in next year's midterm elections.

Its possible single-payer could give way to less sweeping changes if Democrats retake power.

Democrats have revived their push to create a public option a government-run alternative that would be sold alongside private insurance on the ACA exchanges. The idea, which liberals unsuccessfully fought to include in the ACA, would be far less expensive than full single-payer since most Americans would still get coverage from traditional insurance. Another proposal is to allow older people to voluntarily buy into Medicare.

"Every major breakthrough from Civil Rights to Social Security to what happened on the right under Ronald Reagan were driven by significant mobilization behind an idea that was much more extreme than what actually happened," Yale political scientist Jacob Hacker, who popularized the public option, told NBC News.

A few short years ago, Hacker's idea for a public option was killed by conservative Democrats involved in crafting Obamacare who saw it as too radical. Now, Hacker gets attacked by single-payer activists as a sellout for still favoring the idea.

Some politicians are trying to temper expectations. Sen. Ron Wyden, D-Ore., championed the Obamacare provision that allows states to enact their own single-payer plans, but noted that places like California and Vermont have had trouble finding a way to "get from here to there."

Instead, he hinted at a more gradual path to single-payer by passing legislation that would encourage more workers to buy insurance on an individual basis rather than through their employer. If you gave them access to a public option, he argues, it could grow to eventually become the dominant plan.

"You really strengthen the exchanges and probably provide another path for people actually advocating single-payer...to make the transition work," he said.

As for Conyers, who turned 88 a few months ago, hes willing to wait.

"I've said before, this is a civil rights issue and it'll take a movement on the scale of the one Dr. King led," he said. I'm glad we're here it shows we're making progress but my goal isn't a certain number of co-sponsors, it's passing a bill that makes every American Medicare-eligible."

Link:

Government-Run Health Care: Democrats' New Litmus Test - NBCNews.com