This item by J.P. Green was originally published on July 8, 2011.
Nate Silver’s well-reasoned analysis, “Why the Republicans Resist Compromise” at his Five Thirty Eight blog at The New York Times affirms the meme that the GOP is pretty much ensnared by its more conservative faction. While this conclusion is no big shocker to most political observers, Silver’s data-driven analysis, as presented in his chart “Ideological Distribution of People Voting Republican for U.S. House,” is impressive and instructive:

The Republican Party is dependent, to an extent unprecedented in recent political history, on a single ideological group. That group, of course, is conservatives. It isn’t a bad thing to be in favor with conservatives: by some definitions they make up about 40 percent of voters. But the terms ‘Republican’ and ‘conservative’ are growing closer and closer to being synonyms; fewer and fewer nonconservatives vote Republican, and fewer and fewer Republican voters are not conservative.
The chart, culled from exit poll data, shows the ideological disposition of those people who voted Republican for the House of Representatives in the elections of 1984 through 2010. Until fairly recently, about half of the people who voted Republican for Congress (not all of whom are registered Republicans) identified themselves as conservative, and the other half as moderate or, less commonly, liberal. But lately the ratio has been skewing: in last year’s elections, 67 percent of those who voted Republican said they were conservative, up from 58 percent two years earlier and 48 percent ten years ago.

Silver notes the pivotal role of disproportionate conservative turnout in last year’s midterms, and the unfortunate consequences for Dems:

This was fortunate for Republicans, because they lost moderate voters to Democrats by 13 percentage points (and liberals by 82 percentage points). Had the ideological composition of the electorate been the same in 2010 as in 2008 or 2006, the Republicans and Democrats would have split the popular vote for the House about evenly — but as it was, Republicans won the popular vote for the House by about 7 percentage points and gained 63 seats.
Many of the G.O.P. victories last year were extremely close. I calculate that, had the national popular vote been divided evenly, Democrats would have lost just 27 seats instead of 63. Put differently, the majority of Republican gains last year were probably due to changes in relative turnout rather than people changing their minds about which party’s approach they preferred.

Addressing “the enthusiasm gap within the Republican party,” Silver cites a Pew Research poll conducted a few days before the election which indicated that,

Among conservatives who are either registered as Republicans or who lean toward the Republican party, about 3 out of 4 were likely to have voted in 2010, the Pew data indicated. The fraction of likely voters was even higher among those who called themselves “very conservative:” 79 percent.
By contrast, only about half of moderate or liberal Republicans were likely voters, according to Pew’s model. That is about the same as the figure for Democrats generally: — about half of them were likely voters, with little difference among conservative, moderate and liberal Democrats.
So the enthusiasm gap did not so much divide Republicans from Democrats; rather, it divided conservative Republicans from everyone else. According to the Pew data, while 64 percent of all Republicans and Republican-leaning independents identify as conservative, the figure rises to 73 percent for those who actually voted in 2010.

Silver cites data indicating that “Republicans are still fairly unpopular,” but adds,

…As long as conservative Republicans are much more likely to vote than anyone else, the party can fare well despite that unpopularity, as it obviously did in 2010. But it means that Republican members of Congress have a mandate to remain steadfast to the conservatives who are responsible for electing them.
Presidential elections are different: they tend to have a more equivocal turnout. The G.O.P. can turn out its base but it has not converted many other voters to its cause, and President Obama’s approval ratings remain passable although not good. The Republicans will need all their voters to turn out — including their moderates — to be an even-money bet to defeat him.

Silver believes that, if Romney is nominated, he would have a clear shot at turning out the GOP moderates, while Bachmann could alienate enough of them to give Obama victory.
At his TPM Editors blog, Josh Marshall applauds Silver’s analysis of conservative domination of the GOP, but adds that it shouldn’t let Democrats off the hook for their failure to take advantage of it:

When I castigate the Democrats for not having a clear message or President Obama for not having an “outside game” in the debt fight, readers will often write in to say that I’m ignoring the fact that the modern GOP is a coherent and highly ideological party while the Democrats simply are not. So Republicans are inherently more able to function as a unified force with a unified message than the Dems. In fact, these folks will argue, it’s not even right to talk about “the Dems” because that buys into the illusion that they’re a party like the GOP as opposed to a coalition of constituencies.
For my money, I don’t find this a sufficient explanation. I do think the Dems are consistently guilty of what amounts to a political failure — the failure to devise and push a consistent message and play on the weaknesses of their foes. I’ve made these points so often that there’s no need (and probably appetite) for me to restate them here. However, it is important to note these structural realities that create a genuine tilt in the playing field of our politics, one that makes it easier for 35% to 40% of the electorate to dominate the country by having virtually total control over one of the two parties.

“Still,” Marshall concludes, “…Politics matters. And on that count the Dems continue to be captive and captured by a weakness it is in their collective power — and for a president to a great degree individual power — to change.”