Hell, they decide on their own what "the obvious constraints" are.
And the Dems buy into it. How about setting up some of their own "obvious constraints," such as not tolerating any more attacks on social programs or even THINKING about compromising (i.e. caving) with the Republicans?
Obama barely got the thing passed because he kept trying to buddy up to the Republicans, who weren't going to cooperate no matter what. The process was also very non-transparent. There was NO easily available information about the proposal, which let the Republicans run wild with horror stories from British tabloids, even though the Obama plan had nothing in common with the British system.
Here's what a hypothetical leftist Ronald Reagan would have done to introduce single payer. He would have gone on TV, outlined single payer in five sentences or fewer, and told the audience to call, write, or e-mail their Congresscritters if they supported the plan. He would have sent out all the Democratic Congresscritters to talk up the plan in their home districts, using the same five points in town hall meetings and on local media.
Reagan always got what he wanted, and that's how he did it. Just substitute "rightist" for "leftistL and "Republican" for "Democratic."
Obama had a further advantage, which he totally blew. He had millions of eager, idealistic young people (to an extent I have not seen since the days of Robert Kennedy) who would gone door to door and leafletted for health care if he had only asked them.
I voted for Obama, but I lost a lot of respect for him when he didn't even meet with single-payer advocates but met with insurance company executives behind closed doors. I was on DU when Cheney met with the energy company execs behind closed doors, and there was universal indignation and suspicion that he had something nefarious in mind, otherwise, why not have a public meeting? But it was OK for Obama to meet in secret with the insurance company execs? Sorry, but that's blind loyalty. Prioritizing the wishes of the corporations is bad no matter who does it.