Consistent with the earlier research, we found that those House Democrats who voted in favor of ACA ran around six points behind those Democrats who voted against it in the 2010 midterms. 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.
Politicians and political observers often talk about the demands of party ("Sometimes party loyalty asks too much," said John Kennedy), but it's rare that we see such an explicit tradeoff. Nancy Pelosi actively pushed to pass this bill, achieving a goal that the Democratic Party had been pursuing for decades. And the price of the goal was that she lost her speakership and a few dozen of her colleagues lost their jobs.
Now, how exactly did this happen? Why did a vote for ACA cause a Democratic House members' voters to turn against her? We examine this at the level of the individual voter using a CCES survey. The findings suggest that voting for health reform caused voters to perceive a member as being more liberal, even controlling for the members' overall voting record. In the graph below, respondents were asked to evaluate the ideology of their member of Congress. The solid line 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.
A final question, which goes somewhat beyond the paper, is whether the effect of ACA goes beyond the 2010 election. Will it hurt Democrats again this year? Some early polling suggests it might, although Bernstein is skeptical. John Sides gets into this issue in a recent op/ed. One reason ACA might not have much of an effect in Congressional elections this year is that all the vulnerable Democrats were kicked out in 2010. The ACA supporters who remain have relatively safe districts. It may be that a few senators who were not up in 2010 could still pay a price (this may be part of Ben Nelson's reason for retiring), but that's much harder to say.
My co-authors are blogging on this issue, and I'll link as those posts become available. Here's what Sides had to say this morning, and here's a post from Nyhan on this topic. Greene had this to add. We got a bit of pushback from the likes of Jonathan Chait, so here's Sides' excellent followup.