Keith, in his brilliant post below on dictatorship and malignant narcissism, is surely right to say that no one in a republican system – not even a Nixon – can be quite as crazy as a true dictator, because there’s too much in the environment that he doesn’t control. I would emphasize here the importance of free press in keeping officials sane by confronting them with external reality; a Rick Perry Administration might be able to partially tune that out by relying on Fox News, but that would still be a far cry from a world in which nothing is published that departs from the party line.
Still, the difference is in degree, not in tendency. Acton was right that all power corrupts, though only absolute power corrupts absolutely. Karl Deutsch analyzes this problem in The Nerves of Government. I don’t have the book in front of me, but here’s a fairly close paraphrase of the key paragraph:
Learning means adjusting your ideas to fit the world. Power is the capacity to adjust the world to fit your ideas. So power means the ability not to learn from your mistakes.
Footnote The phenomenon is not new: think of Tiberias, Nero, or Caligula. (The history of European monarchy doesn’t include many such characters: neither Henry VIII nor Louis XIV developed into a Tiberias. Does a formally hereditary system breed fewer monsters? Or was it the relatively limited penetrative capacity of the early modern state, combined with the existence of aristocracies, chartered towns, the Church, and other imperfectly controllable power centers?)
To be stable, a political system needs to develop a mechanism that limits the malignant narcissism of its rulers. A party dictatorship has an advantage over an individual dictatorship in this regard: none of the individuals who constitute the Chinese Politboro has the power to rape women at random. Still, the system as a whole is still likely to be deficient in reality-checking. That’s an argument for placing a cautious bet on India in its competition with China.