An email: “Interesting theory re the missing ‘middle lane’ in the Republican party, but what it doesn’t explain is why the candidate in the left lane usually wins. It can’t just be a coincidence that we get Doles and McCains. . . .”
I’ve taken a stab at an explanation for that before. I think the main explanation is that the party establishment tends to consolidate earlier than the most conservative Republicans do. Also, the most conservative Republican voters often back candidates that are unacceptable to the middle of the party, for example because they lack political or executive experience. The 1996 race is a pretty good case. Dole might have lost to Phil Gramm in a one-on-one race putting a relatively moderate against a relatively conservative senator. But Pat Buchanan and Steve Forbes made it impossible for Gramm to unify the center-right of the party: Social conservatives went to Buchanan and economic conservatives to Forbes, and Gramm got knocked out early. The eventual choice was Dole or Buchanan. The party was not in love with Dole but was not going to go with a controversial columnist with no elective experience.
[Update: I fixed a typo.]