Voting for President Obama's healthcare reform law cost Democratic incumbents 5.8 percentage points of support at the polls in 2010, according to a new study in the journal American Politics Research.
The study helps explain why Democrats lost 66 House seats, significantly more than the median academic forecast of 44 to 45 seats, study co-author Brendan Nyhan of Dartmouth College writes on his blog.
"We show that the roll-call effect on vote share was driven by healthcare reform. Democratic incumbents who voted yes performed significantly worse than those who did not," Nyhan writes. "We then provide simulation evidence suggesting that Democrats would win approximately 25 more seats if those in competitive districts had voted no, which accounts for the gap between the academic forecasts and the observed outcomes."
The study doesn't offer much insight into what might happen this November, however.
"The economy is the dominant issue in the presidential race" this time around, Nyhan writes, "and most of the vulnerable Democratic incumbents in Congress lost in 2010."