This is one of Nate Silver's most interesting posts in a while, and I encourage you to read the whole thing. But to summarize his findings:
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.
Nate presents this trend in stark graphical terms:
While Nate initially frames his post around the question of why the GOP is utterly recalcitrant when it comes to negotiations over the debt ceiling, what he ultimately drives at is why the increased conservative-ization of the Republican electorate could help give someone like Michele Bachmann the Republican nomination:
Imagine that Ms. Bachmann has won the Iowa caucuses while Mr. Romney has taken the New Hampshire primary, and the nomination is essentially up for grabs between them. As the contest shifts to a key state like Ohio or Pennsylvania, suppose that conservative Republicans split 60-30 in Ms. Bachmann’s favor (with a few voters opting for a hanger-on like Ron Paul), while moderate Republicans go 80-15 for Mr. Romney. Who is going to win?
Turnout would be decisive. If two conservative Republicans cast ballots for every moderate Republican — roughly the ratio in 2008 — Mr. Romney would prevail by a couple of points. But if the turnout looks more like 2010, and there are three conservative Republicans at the polls for every moderate Republican, Ms. Bachmann would win by about six percentage points.
There are also longer-term electoral implications to all of this which Nate did not address in his post—issues which go beyond the immediate potential fallout of a Bachmann candidacy. While 2010's successes appeared to give the GOP a new lease on life, I believe it was just that, a lease—and leases are by definition temporary. As Republican primary voters continue to insist on more and more conservative candidates, those candidates will have a harder and harder time winning majorities, whether for Congress or the presidency.
This was a common strain of thought after Obama's big victory in 2008, but it got largely shelved once Republican fortunes started to turn around in 2009. I maintain, though, that last year's red surge was due, above all else, to the disastrous economy and not any permanent turnaround for the GOP brand. The Republican tent is only getting smaller, as Silver's calculations show. While this will have sharply negative consequences for the country in the near term, I think it may present even harsher consequences for the Republican Party's future over the long run.