Nate Silver answers this question in the affirmative:

The Republican Party is dependent, to an extent unprecedented in recent political history, on a single ideological group. That group, of course, is conservatives.

He shows that, among voters who voted for a Republican House candidate, the percentage who were conservative increased from 58% in 2008 to 67% in 2010.  He suggests that this reflects an “enthusiasm gap” in turnout—conservative Republicans were more likely to vote than other Republicans—and that this helps explain why Republican leaders are reluctant to compromise with Obama.

I’m not completely sold on the enthusiasm gap as an explanation for 2010.  Just based on the number of seats they had to defend, the state of the economy, and Obama’s approval rating, they were forecast to lose 56 seats, according to Gary Jacobson.   Moreover, the preferences of so many demographic groups shifted against the Democrats (see slides 9 and 10) that I don’t think it’s just about, or even primarily about, turnout.  See also my earlier post.

But here’s a more important caveat: Republican members of Congress are more conservative than Republican voters.  In fact, representatives from both parties are more ideologically extreme than voters in their parties.  That’s the conclusion of a recently published paper by Joseph Bafumi and Michael Herron (gated; ungated).   In their study of the 109th and 110th Congresses, they use a large national survey that asked voters their positions on key roll call votes taken in Congress.  Thus, Bafumi and Herron can measure the ideologies of voters and members on the same scale.  Once they do so, representatives emerge as far more extreme.

You might think that, even if a Republican representative is more extreme than a Republican voter, he or she might not be more conservative than a conservative Republican voter, who are the focus on Silver’s post.  Bafumi and Herron shed some light on this as well by focusing on a more ideologically extreme group of voters: people who donate money to candidates.  But even most donors are still not as ideologically extreme as the candidates.  For example, only 13% of Republican donors were more conservative than their representatives in the 109th and 110th Congresses.  To be sure, donors are ideologically closer to representatives than are non-donors, but representatives are still more extreme.

Of course, it is true that the 112th Congress is not the 110th Congress.  But I doubt that, in the past 4 years, Republican voters became so much more conservative that they actually matched Republican representatives.  Indeed, all available evidence suggests that the parties in Congress are polarizing much more quickly than the partisans in the public.  It is true that there are fewer liberal or moderate Republicans and conservative Democrats in the public than there once were—you can explore some data here—but the public doesn’t look anything like Congress.   In part, this reflects the lack of ideological fidelity within the parties.  To wit: Silver’s results suggest a shift in the fraction of people who identify as conservative, but even conservatives don’t want to cut most kinds of government spending.

In general, I don’t think we get very far attributing the Republicans’ reluctance to compromise on the budget to what their constituents want.  It likely stems much more from the opinions of interest groups and activists, which have become ideologically polarized (pdf) and which play a crucial role in selecting candidates.  Or it stems simply from the ideologies of members themselves.  People underestimate how much the behavior of politicians is sincere.

[Cross-posted at The Monkey Cage]

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John Sides is an associate professor of political science at George Washington University.