We conducted thousands of simulations and found that, in the majority of simulations, Democrats retained at least 25 additional seats if they had all voted against ACA. That's enough for them to have held the majority…The solid line [in the above chart] represents voters in the districts of Democratic House members who voted against ACA; the dotted line represents those in the districts of ACA supporters. That one yes vote had an enormous effect on voters, causing them to perceive their representative as being substantially further to the left. And as numerous studies have shown, being ideologically extreme tends to reduce one's vote share.
[N]one of this is to say that Democrats should have declined to pursue health care reform, which was arguably their party's top policy priority after the 2008 election. Parties are frequently willing to pay an electoral penalty to enact their preferred policy agenda. What our analysis shows, however, is that the costs of passing the legislation were significant.