The answer to the “puzzle” is simple. It is for the American people to begin to look exclusively at, and judge candidates solely by, adherence to the first principles of the republic, not something so trivial as a mere party label.
Primaries are the time for the interesting action.
America’s response to finding the Mr. Wonderful most likely positioned to beat Barack Obama (whether we like it or not that was within the GOP) was a monstrous MEH.