Publius makes some very shrewd observations:
The fear [of Huckabee from the right] is not of fundamentalism itself. The fear is losing. On some level, they know that these positions freak out mainstream America. That's why Bush fumbled about on Roe v. Wade during the 2004 debates and could only bring himself to speak in code (Dred Scott). The backlash shows that these people - like much of the GOP establishment - are ashamed of this coalition. They're happy to make out with them behind the football bleachers on Saturday night, but ignore them in the lunchroom on Monday.
But still, the GOP needs them. And so the deal has been that "the crazies" stay below-radar in return for below-radar policies (e.g., judges, denying funding to international organizations who don't believe in abstinence-based policies). Huckabee is a threat because he violates the terms of the unspoken deal.