Why did Democrats do so poorly in the 2010 elections? The median academic forecast was 44 to 45 seats (PDF). However, Republicans significantly outperformed expectations in picking up 66 seats in the House and six seats in the Senate.
After the election, John Sides, Eric McGhee, and I found that Democratic incumbents who voted the most controversial legislation of the 2008-2010 period -- TARP, the stimulus, cap-and-trade, and health care reform -- performed significantly worse than those who voted no on those bills. Seth Masket and Steven Greene reached a similar conclusion about the effects of supporting health care reform on the vote share received by the most conservative House Democrats who ran for re-election.
Afterward, we joined forces Voltron-style and produced a new article (gated; ungated) that is forthcoming in a special issue of American Politics Research on the 2010 election. In it, we show that the roll call effect on vote share was driven by health care reform. Democratic incumbents who voted yes performed significantly worse than those who did not. Even among a more comparable set of members and districts that we isolate using statistical matching procedures, the estimated effect of support is -5.8 percentage points. 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.*
Why did health care reform have such dramatic effects? Individual-level survey data shows that health care reform supporters were seen as more liberal and thus more ideologically distant from voters.
Using mediation analysis, we then show that perceived ideological distance appears to be the key mechanism linking incumbent support for health care reform with individual-level opposition among their constituents. In short, support for health care reform is associated with greater perceived ideological distance, which in turn is associated with a reduced likelihood of supporting the incumbent.
Going forward, the implications for 2012 are less clear. As Sides notes, the economy is the dominant issue in the presidential race and most of the vulnerable Democratic incumbents in Congress lost in 2010. Nonetheless, it will be interesting to see whether challengers can successfully target any remaining Democrats in competitive districts or states who supported health care reform. Will it hurt the party again?
Of course, none 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.
* As Kevin Collins and Jonathan Chait point out, it's possible that news coverage and GOP ads would have focused on the other roll call votes and that they would have had correspondingly greater effects on vulnerable Democrats. In that case, the net effect of supporting health care reform on Democratic losses in 2010 might have been reduced or eliminated. For more on these counterfactuals, see the responses from John Sides and Eric McGhee. See also Seth Masket on how some media coverage has distorted our findings.