What Huntsman means by tacking to “the middle” on immigration and environment is to take up positions that have been almost universally rejected within his party. On immigration, Huntsman is advocating moving away from a solid majority of the public, and there are hardly any voters who base their voting largely or primarily on environmental issues. This is not just a matter of a “moderate” being at odds with a more “conservative” party base. Relatively speaking, almost every Republican nominee has been much more “moderate” than the party base, so that would not be anything new. Huntsman’s case is different.
Huntsman has staked out positions that have little or no support anywhere in his party. Even most “reformist” conservative wonks are unsympathetic to liberalizing immigration policy, in part because they correctly see mass immigration of unskilled laborers as something that exacerbates social and economic inequality.
That last point is debatable. But all of this is premised on the notion that current Republican shibboleths - no tax hikes ever, climate-change is a hoax, mass immigration is a problem, we need to stay longer in Afghanistan - will not collapse even in the ideologized brains of the GOP base by the time Huntsman actually does run - in 2016. Huntsman merely has reality to wait for - like most actual conservatives.