Perceptive Jay Cost writes persuasively on Real Clear Politics that the process and passage of healthcare legislation is what has turned the tide against House Democrats:

Partisans on both sides tell themselves stories about why they're up, why they're down, and why the other side is where it is. These stories usually contain at least a grain of truth, but they also help encourage ideologues in the face of an impending rejection by the electorate. Democrats ignored the political problem of health care in the fall and winter -- arguing that Martha Coakley and Creigh Deeds were bad candidates, that voters had been turned off by the health care bill because of the process, and that they would come around once the many benefits kicked in. Now, they're pointing to the economy as the only significant reason why the party is in trouble.

It would be difficult for any strong partisan to admit that such an accomplishment was so deeply unpopular. Yet the polling is pretty unequivocal on the relationship between the Democrats' fortunes and the health care bill. 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.

Reconstructing the Democrats' meme, we can fairly say that the economy is a huge problem for the party. Of this, there can be no doubt. We can also say that the stalled recovery denied the Democrats a chance to win back the voters they lost over health care. But the process and passage of health care reform were crucial elements in the story. That's when the party started losing the voters it needs to retain control of the government.