As I watch the sorry spectacle that used to be American conservatism continue its descent into absurdity, I think that "left" vs. "right" is a much less meaningful distinction today than "reality-based" vs. "reality-blind".  It is deeply disturbing to see adult citizens (voters!) who demonstrate no logical coherence or intellectual openness at all in their deeply held beliefs on taxation, government spending, economic fundamentals, climate science, evolution, religion, race, culture or even the history of the United States itself.  Ideological blindness is not limited to the political right - it has been found (and continues to exist) at all extremes of the political spectrum.  But at the present time in our country, the political right seems to be overloaded with willful ignorance.  This can be morbidly entertaining but it is actually a very bad thing.  Government policy-making would benefit from serious, thoughtful conservative input; effective single-party rule will inevitably lead to corruption and waste; our democracy itself can be threatened if the willfully ignorant through some unforeseen circumstance achieve political power.  

How did this happen?  What "broke" the right (and could the left "break" for the same reasons)?  I'd guess this subject has been written on already; here are some theories.

1.  Over-reliance on issue-based polarized voters

Political parties exist to win elections.  They spend money to convince voters to support their candidates - they win when they beat the other party in cost-per-vote.  Highly polarized voters - motivated partisans who would never vote for the other guys - are valuable because their cost-per-vote is very low.  Both major parties pursue polarizing strategies with various subgroups, seeking to maintain a "base" of loyal partisans with low cost-per-vote.  

Republicans and Democrats have both seen success with "identity polarization".  To be white and Southern is to be Republican.  To be African-American is to be Democratic.  These alignments are deeply rooted in individual identity and can mostly be relied upon even when parties pursue policies that are against the interests of the polarized groups.

Republicans have been much more successful than Democrats in "issue polarization".  Abortion, gay rights and gun control are all highly polarizing issues; opponents of each are much more likely to be "single-issue" voters.  Republicans seized upon this and widened their polarized "base" with aggressive positions on these issues.  However, this comes at a price:  a loss of flexibility around these issues.  Identity-polarized voters will live in a "big tent"; issue-polarized voters will not.

2.  Agenda conflict with right-wing media leaders

The media landscape has changed enormously over the past three decades, and a strong right-wing media has emerged - talk radio and Fox News.  Within right-wing media, certain individuals - Rush, Hannity and others - have established themselves as strong opinion leaders with a sizable audience.  This has largely benefited the Republican party, giving them a "propaganda arm" to build support for right-wing positions and demonize political opponents.  But this is a marriage of convenience for right-wing opinion leaders:  their agenda is to make money, not to win elections.  

There are left-leaning opinion leaders with large audiences too:  Keith Olbermann, Michael Moore, John Stewart.  But the Democratic party has so far not made the mistake of entangling its electoral agenda with these figures.  By contrast, the Republicans - at both a party and individual level - have sought out the endorsement of right-wing opinion leaders.  This is a winning strategy when partisan fevers are running high and the country is tilting rightwards, but it's a disaster when the party needs to seek the center.

Centrism is not where opinion leaders make money.  They thrive on controversy and extreme opinion - this is what sells ad time and books.  Rush would bite the heads off live guinea pigs if he could make enough money from it; he may prefer Republican government, but he won't help Republicans win elections if it costs him advertisers or audience.  The entanglment of Republican politics with right-wing media and its opinion leaders is preventing Republicans from staking out centrist positions.

3.  Failure of key "big ideas"

Conservative intellectuals were buoyed by successes in the 1980s-90s.  The economy was booming under supply-side economics; the Soviet Union collapsed when the US took a more confrontational, muscular approach to foreign policy.  These two "big ideas" were central to modern conservative thought:  deregulation and lower taxes to fuel economic growth; an aggressive, hard-power foreign policy to build a more free and peaceful world.  Both ideas were pushed too far and led to catastrophe.

When the intellectual underpinnings of a movement are so badly discredited, the intellectuals hang their heads and shut their mouths.  They don't want to say that the ideas were just plain wrong (and they weren't; they were just pushed too far).  But they don't have strong new ideas to put forward, and they don't want to try to defend the old ideas when there is so much evidence that they brought about disaster.  So they stay quietly out of sight.

Unfortunately it is the thoughtful intellectuals in the conservative movement who are badly needed by Republicans right now to shape more centrist views and sell them.  With these guys off the stage, the only voices being heard are the loud and unreasonable ones, spouting the old ideologies in more partisan terms.  

Will this situation fix itself over time?  Possibly.  My worry is that single-party rule by Democrats will lead to corruption and scandal, angering independent voters, and that Republicans will get back into power (at least partially) before they've worked out these problems.