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.

Democrats in the lead-up to the elections took a number of tough votes — notably on the Wall Street bailout, the stimulus and cap-and-trade — but none was as unpopular as their support for the healthcare reform law.

“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.”