Another way to make the same point would be to say that the post-Reagan Republican Party may be the more doctrinaire major party, but it’s still a complex institution organized around the imperative of winning elections. To that end, it still empowers an ideologically flexible party establishment (made up of incumbent politicians, party functionaries, special interests and large bundlers) to winnow the field of viable presidential candidates down to the two or so most electable candidates.
Very occasionally, the Republican establishment bungles the job, as it did in 1964 when it couldn’t unite behind a candidate who could stop Barry Goldwater from winning the presidential nomination. But that, as far as I'm concerned, is the exception that proves the rule. This is still the party that nominated George H.W. Bush in 1988, Bob Dole in 1996 and John McCain in 2008. If "moderate" Republican nominees are getting more conservative over time it's less because the Republican establishment is getting weaker relative to a more doctrinaire party base than because the median voter in the electorate as a whole has been getting more conservative.
If what I’m saying amounts to anything, I should be able to specify in advance what it would take to persuade me that I’m wrong. So let me offer this: seeing the Republican establishment get behind a presidential candidate who visibly lacks the will or the capacity to modulate his message between now and 2012 would go a long way toward showing that the modern Republican Party is no longer the animal I'm presuming it is.
You could extract a pretty good test of my thesis from Jonathan Chait’s plausible observations about the possibility of a Paul Ryan presidential candidacy (my emphasis):
“One story I've been following but haven't written about is the possibility that Paul Ryan might decide to run for president. When you have the power to set your party's vision of government for the next fifty years, and nobody in the party is allowed to disagree with you, or even dodge paying fealty to you, then you already are the party leader. Ryan's disavowals of interest never struck me as terribly strong. Now he's dropping even stronger hints . . .If Chait's right, I’m either wrong about the Republican Party or saying something trivial.
“[T]his strikes me as a clear signal that Ryan is at least considering a presidential run. And if he runs, I believe he would immediately become the front-runner, and perhaps even the runaway front runner. He is adored by party activists and elites in equal measure and is the embodiment of the party consensus.”