I do not think these two are comparable. Goldwater was riding on- and pandering to- a single issue - Southern racism and the Federal government meddling with it (which is one coin with two faces since the Civil War). The subsequent popularity of the movement he initiated was due not as much to his seriousness (or wackiness) but to something over which he had little influence - the stance of the Democratic administrations on the race issues (busing, great society etc.). The Repugs simply gained from a windfall caused by popular reaction to this stance. Was not it Johnson who once commented that he delivered the South to GOP?
By contrast, the Dean constituencies are not as united as those who supported the Goldwater movement. There are folks with very diverse interests and agendas that are often at odd form one another - from criminal justice reform, to racism and poverty alleviation, to health care, to environment protection, to urban renewal, to anti-imperialism, and to a myriad of conflicting ethnic-specific issues. Pulling a cohesive movement of these diverse constituencies along the lines that Goldwater did is not an easy task, to say the least.
Another issue is the ability of to tap into popular sentiments, something that Democrats are often thought to be very poor at vis a vis the Repugs. But such opinions miss one important point - US is not an elective monarchy but a democracy or plutocracy. The power of politicians is constrained by business and corporate interests.
Thus, a racist movement against "encroachments" of the Federal government is not going to alarm the business interest at all, au contraire it may even serve some of their interest. By contrast, the issues raised by Dean (or Kucinich) constituencies ring all possible alarm bells in the corporate boardrooms. As a result, the corporate plutocracy is likely to remain neutral or even mildly supportive of a Goldwater-like movement, but will fight tooth and nail the agenda proposed by popular movements that usually associate themselves with progressive Democrats.
This balance of power is a no brainer - anyone interested in being elected to a political office - as opposed to making a grandstanding of a purely symbolic nature - knows that and must tread very lightly not to disturb the corporate attack dogs too much. As Johns Dewey aptly observed, politics is a shadow cast on society by big business, and as long at this is so, the attenuation of the shadow will not change the substance. Or more accurately, the attenuation of the shadow may move things one way - away from the areas of concern to big business (usually to the right, but not necessarily) - but not in the other direction.
Anyone who wants a real political change in this country should first figure out how to break the iron grip of the big business interests on this society, and only then concentrate on organizing political campaigns to run this country's institutions in a more progressive and just fashion.
Wojtek