The conservatives denounce their presidents for the same reason that the left denounces Stalin: they want to evade responsibility for the results of the policies imposed by monsters that they themselves created. When the left does this, we know not to take it too seriously. If you give the state the right to expropriate all private property, you can't be too surprised when dictators take over.
Similarly, when the whole of your intellectual enterprise has been wrapped up in celebrating the nation-state and its wars, condemning civil liberties, casting aspersions on religious liberty, and heralding the jail and the electric chair as the answer to all of society's problems, you can't complain when your policies produce tin-pot despotic imperialists like Bush. You have no intellectual apparatus with which to beat them back.
The problem with American conservatism is that it hates the left more than the state, loves the past more than liberty, feels a greater attachment to nationalism than to the idea of self-determination, believes brute force is the answer to all social problems, and thinks it is better to impose truth rather than risk losing one's soul to heresy. It has never understood the idea of freedom as a self-ordering principle of society. It has never seen the state as the enemy of what conservatives purport to favor. It has always looked to presidential power as the saving grace of what is right and true about America.
Along with my general approval, I note that any ideology, including libertarianism, can be a powerful shield from the truth.