Election 2010: How marginal was the messaging?

Today Brendan Nyhan hammers home his signature theme: presidents don't move public opinion by speechifying.  He has been relentless about this for years. Reagan couldn't move the public to support the Contras; Bush couldn't do it for social security privatization; and Obama didn't do it with his Sept. '09 speech promoting health care reform.

A natural corollary is that elections are determined mainly by structural factors: the state of the economy, the number of seats the in-party has to defend, the proportion of those seats that are in the opposing party's traditional territory, etc.. On the eve of the last election, Nyhan had a memorable post cataloging every oft-recited narrative about Obama's imagined failures of messaging or strategy, with links to past posts debunking most of them.

As it turned out, the Republicans out-performed the structural models, the most commonly cited of which, by Douglas Hibbs, forecast a gain of about 45 seats. Why?  Aspects of the current economic woe that the model could not capture?  Extraordinary GOP messaging that maximized the structural advantage? 

On November 11, Nyhan and colleagues Eric McGhee and John Sides published an Election Postmortem reporting some preliminary numbers crunching. No dominant explanation emerged.The Tea Party's impact seems to have been marginal. Money was not decisive. Structural factors explained much, but not the size of the victory margin.

The authors did find one factor, though, that raises more questions than it answers, and that I found astonishing on its face: that most House Democrats paid dearly for every 'yes' vote they cast on major legislation.  The writeup of this finding should be digested in full: