That pattern in history is the reason I think it's perfecty fair and a good idea to look at the fringe groups. I think that's the whole point behind the article putting so much attention on that one Pam Stout. It was a demonstration of how you have a person who moderate strictly through being a-political, an "ordinary person", that winds up subscribing to very eccentric beliefs as she gets caught up in the hysteria.
Whether or not these sorts of movements get swallowed up by the extremists is all a matter of how good conditions are, and how the mainstream is in turn perceived. If the economy and other factors just get worse, the establishment looks worse, even more moderates will turn away out of sheer practical discontent, and the extremists have more fodder. If things get better, most of the moderates lose the practical underlying reasons they moved away from the establishment anyhow, and the insurgent movement shrinks and is left with only with crazies that no longer sound credible. Note the comment in the article, that in these bad times, conspiracy theories stop sounding so paranoid. The confusion caused by social stress!
I think there are only three things that happen to a movement like the tea part movement. 1) It dies shortly, save some longterm stragglers, and ends up being a fad without accomplishing things. 2) Its ranks swell, the process I stated above follows, and some commanding extremist ideology makes it a dangerous movement. 3) It becomes co-opted by the establishment, and may remain aesthetically or minorly shift the paradigm of the establishment, but in practical terms, mostly just gets devoured by the same old stuff.