Jay Cost at Real Clear Politics continues to argue that health care is a huge part of the cause of Democrats' political difficulties:
The Democrats' control of the House did not become tenuous recently. At best, some of the more immediate warning signs - e.g. individual incumbents like Betty Sutton now appear to be in jeopardy - have manifested themselves recently. But there has been a real danger of losing the House for some time, a danger that predates "Recovery Summer" and goes back to the health care debate. ...
It was during the health care debate that the essential building block of the Democratic majority - Independent voters - began to crumble. It was evident in the generic ballot. It was evident in the President's job approval numbers. It was evident in Virginia, New Jersey, and Massachusetts.
It's obviously true that the Democrats lost a lot of support "during the health care debate." The health care debate took about a year. My argument is that, during a period in which unemployment was rising and the Democrats controlled the entire government, Democrats would have bled support regardless of what they were debating. If they declined to carry out their campaign promises, they would have lost support. If they cooperated with Republicans to continue or deepen Bush-era tax cuts for the rich -- the only policy upon which bipartisan cooperation was possible -- they may have bled somewhat less support because people like bipartisanship, but it would have been terrible policy.
You can make some counter-factual argument that never attempting to pass health care would have been a good political alternative, although you have to account for the massive liberal firestorm this would have provoked. You can make a better argument that passing health care quickly instead of spend month after month sitting on Olympia Snowe's doorstep would have been a shrewder plan. I think the conservative argument that, after investing months and months into health care, taking high profile votes in both chambers, it would have been shrewd to then abandon the whole thing to failure is transparently unconvincing. That's a recipe for absorbing almost all the costs of passing health care reform, getting none of the benefits, and driving your base wild with rage at you.
Of course, I can't prove that counter-factual, either. None of these counter-factuals is something that you can prove. But the method of saying that Democrats lost support during (very long) event X, therefore (very long) event X caused them to lose support, is not a persuasive argument.