I assume I can't really blame William Galston for the headline of his TNR piece arguing that ACA sunk the Democrats, but while the article happily doesn't use the word "proof", it is at any rate not a conclusive argument.
The real answer, I'm afraid, is that we don't know the effects of health care reform on the elections.
I've posted about this several times, but it's a pretty important point. There's no real way to prove the effect of health care reform on the 2010 elections, one way or another.
To begin with, Galston relies on surveys of voter intent about health care. He notes that many self-identified independents (who certainly flipped from D to R in 2010) register strong opposition to health care reform, and also that those who identify health care reform as important to their vote broke sharply against Democrats.
The problem with that sort of evidence is that, as John Sides mentioned recently, people are poor judges of the reasons they do things, most certainly including voting. One thing we do know is that there have been plenty of attack ads and commotion in the Republican partisan press about health care. What we don't know from the surveys Galston cites, then, is whether people who otherwise would have voted for Democrats turned to the GOP because of ACA -- or whether people who had decided to vote GOP for other reasons (such as the economy, for example) turned to ACA as an explanation, since thanks to the ads it was convenient.
Now, fortunately, we do have some evidence on one, narrower question: all else equal, how did a vote for health care reform affect incumbent Democrats? It's evidence, not proof (these things are difficult to model correctly for quantitative analysis, so different researchers can have someone different findings). But Eric McGhee and John Sides do come up with an estimate that a "yes" vote cost about 4.5 points.
However, I'd point out that the "all else equal" here really matters, and that the data give us little guidance on evaluating the "all else" if it wasn't equal. What McGhee and Sides (or a similar analysis by Seth Masket and Steve Greene) show are the effects of voting yes compared voting no to the final passage vote in the House in Spring 2010, assuming that the bill passed. That leaves a lot of questions, however.
What if a handful of Democrats flipped to "no" (thus improving their election chances a bit) but the bill failed? We can't really know what would happen next. Would the president's approval rating been hit hard, as Bill Clinton's may have been after his health care initiative failed in 1994? What would that have done to Democratic chances in November? We don't know. It's hard for me to imagine that Barack Obama would be more popular if he had done everything the same on health care but lost. And we do know that the president's approval rating does affect House races. So maybe the people who switch to "no" gain a few points, but the entire Democratic ticket loses...how much? We don't know.
There's more. If the bill failed in a final vote because some Members who actually voted for it had instead voted no, also else equal, then some of those Members would also be carrying on their record a fall 2009 vote for passage of the original House bill. Would they be safe because they voted against it in 2010, or in trouble from the earlier vote? That's also a problem if we choose another path, which would be a decision to abandon health care reform after the fall House vote but before the final vote (say, after Scott Brown's election).
Another possibility would have been a decision to abandon health care reform after August 2009. What would have been the effect of that decision? To the extent that attacks on Barack Obama and the Democrats were in fact caused by health care reform -- a very dubious proposition in my view, but if they were -- September 2009 would probably have been too late to do anything about it. No, Democratic Members of the House would not have "yes" votes to defend, but that would not have prevented Republicans from using the issue in attack ads. After all, almost every Democrat in Congress had said nice things about health care reform at some point, and a September 2009 collapse would yield a nice story line about liberal Democrats who tried but failed to do nefarious things about health care.
Moreover, would the Democrats really want to abandon a major initiative after a few rowdy Town Hall meetings? Reputation matters, and a party that allows itself to be intimidated that easily once would be vulnerable for a long time.
Now, what if Democrats had decided to pull the plug after realizing the size of the stimulus, back in early 2009? No bill to committee; do S-CHIP expansion, and maybe a couple other small things, and call it a day. Would that have saved them? Again, I have no idea. They certainly could have avoided attacks on health care. However, it's virtually certain that Republicans would simply have substituted a different set of attacks -- on the stimulus, on "bailouts", on cap and trade, on spending, on deficits. Since those (additional) ads never ran, we can't tell what effect they would have had.
Gaming out this kind of counterfactual is fun, but hardly conclusive. How would Democrats have reacted to everything else over the last two years without the passage of the ACA? What else moves to the legislative calendar with health care gone (hard for me to believe that immigration or energy/environment helps the Dems; more state aid and other stimulus certainly might have, but how likely was that)?
Of course, there's another set of counterfactuals that we can't really test. What if instead of financing the thing through savings in Medicare Advantages, the Democrats had chosen to finance it through massive deficit spending, and while they were at it immediately closed the donut hole? Would that have blunted attacks targeting seniors? Given that virtually no one among swing voters and neutral opinion leaders gave the Democrats any credit for responsible budgeting, would they have been hurt worse if they had actually been massively irresponsible (yes, I know Andrew Sullivan and a few others gave them credit, but very few budget scolds did). I suppose I should add that we also don't know what would have happened had marginal Democrats who were going to vote for the thing anyway had decided in for a dime, in for a dollar and supported a strong public option. Would the costs, if any, in even more virulent opposition have been balanced by considerably more enthusiasm among liberals? I don't know, but it isn't implausible (especially since the public option scored well, and therefore would have allowed them to put more otherwise irresponsible goodies into the bill).
I've always believed, and see no new evidence that points the other way, that once the bill came to the House floor in 2009, the Democrats had little choice but to pass the thing. I strongly suspect the same thing is true as far back as midsummer 2009. Before that, I think Barack Obama and Congressional Democrats had a lot more flexibility -- but there are at least as many reasons to believe that dropping health care reform might have been just as damaging as moving ahead with it.
One way or another, however, the postelection surveys just can't tell us what Galston wants them to.