Leaving aside the merits of Obamacare, it was the case that many Democrats were scared that if they voted for it they would lose their seats, and many did lose their seats, although that vote might not have been the cause.
Again, I don't want to debate the merits of Obamacare, but if you're passing something based on the idea that it is broadly "good," something's going wrong - substance, messaging, timing - if you're nonetheless scared it will be very unpopular.