Although it may be comforting to Democrats to think that their perceived lack of patiotism is the result of a vision that is "too high-minded or too subtle" for the masses to digest, there is, I think, a more likely explanation.
Indeed, perhap Professor Greenberg would have encountered it himself if he had focused more on policy and less on plumbing the depths of archives looking for the facile, symbolic gestures of candidates past and present.
If patriotism is defined as love or loyalty to one's country. We ought to be intellectually mature enough to recognize that the more critical political party should be seen as less patriotic and that patriotism is not necessarily a good thing in all contexts. If Democrats, particularly the left wing of the Democratic party, believe that the United States is way too capitalistic, way too militaristic, way too enamored with the gun culture, or focuses too much on the individual, then why should they consider themselves patriots. For that matter, why should they demand that others see them as patriots.
These are precisely the traits which distinguish us from the many countries on the European continent. They are also the policy traits which, generally speaking, divide the two politcal parties. Make no mistake, if the Republican party were to become extinct, the U.S. would look a lot like France in pretty short order with the exception of our poor schools and no nuclear power.
If Democrats believe that the U.S. should start behaving much more like other countries, they may be right or they may be wrong. However, they should not butcher the language and insist that they be considered as patriotic, if not more so, than their Republican brethren who consistently fight for those distinguishing features.
Grown-ups learn to make choices and discover that sometimes many of the concepts they hold dear do, in fact, contradict one another. If I beleived that I was in a quasi-fascistic state, like so many on the left say they believe, I would consider it my moral duty to be a revolutionary rather than a patriot or loyalist.