As I mentioned to JJV after he posted this, it's a good thing he does not live in Gondor, as Denethor would most likely summon a host of attorneys to slap our esteemed colleague with a libel suit or some such.
CRH has addressed the issue of the various strawmen willed into sluggish life by JJV, so I will concentrate for the moment on the more important subject: Denethor's position in the political structure of Gondor. (You may want to read the next section out loud in your best impression of this guy.)
Simply put, Denethor felt "entitled to Power" (as JJV put it) because he in fact was entitled to it as Steward of Gondor. The Stewards had ruled Gondor for centuries, and it was considered so unlikely that the monarchy would be restored that "when the King returns" had become a colloquialism for something that would never happen.
It is also worth noting that Aragorn's claim to the throne of Gondor was not 100% ironclad (mithrilclad?) in the context of the whole history of Gondor and its relations with the northern kingdom of Arnor, from whose kings Aragorn descended. If memory serves, there was in fact armed conflict between the two kingdoms stemming from a claim of the Northern kings to the throne of Gondor. You can bet that Denethor had his lawyers working 24/7 on that one.
That being said, Denethor most likely knew the true identity of Aragorn and most likely resented the loss of such a powerful position. Who wouldn't? (OK, Faramir, but you would think Tolkien would have spread the altruism around a bit more.) It must also be said that Denethor had fallen under the spell of Sauron via the Palantir of Minas Tirith, so he's lucky there wasn't the equivalent of the 25th Amendment back then. I will save JJV some time by saying that the Palantir is NOT analogous to Le Monde.
Two final points:
1) Although I am mildly surprised that no one else in the geek section of the Right's fever swamps came up with it previously, there is no doubt that JJV came up with the bizarre formulation "Denethor Democrats" first. It has nothing to recommend it outside of its alliterative qualities, so I imagine it will take off among JJV's ilk like one of Gandalf's fireworks.
2) Barone declares that "the elite young men who declined to serve in the military during the Vietnam War set out to write a narrative in which they, rather than those who obeyed the call to duty, were the heroes." This is absolutely correct, though in a slightly different way than Barone intended. 2004 campaign, anyone?