In the late 1970s and early 1980s, the British Labour Party was seen by many as having retreated from the centre ground of British politics into an obsession with left-wing ideological purity. In 1981, a significant number of centrist members left to join the breakaway SDP, pushing the balance of the party's remaining membership further to the left by default. At the same time, a Trotskyist group called Militant was alleged to have been using entryist tactics to expand its influence within the party.
Labour's 1983 manifesto1, popularly known as "The longest suicide note in history", was an unusually detailed document, containing many policy positions considered radically outside the mainstream of popular opinion at the time, and expressed in language which appealed more to the hard left than the general public.
Labour suffered a massive electoral defeat in 1983, partly as a result of votes lost to the SDP, and conventional wisdom in the UK holds that its return to electability was won only as a result of its shift during the late 1980s and early 1990s to a more centrist position (which, among other things, involved the expulsion of many known and suspected Militant members from the party).
I'd like to know:
Is there a name for the phenomenon of a movement or party accelerating away from the mainstream as "moderate" supporters leave and previously "fringe"2 elements seek to gain influence? I've used "radicalised" in the title of this question, but it seems insufficient.
Is the phenomenon always, or even usually, associated with a loss of wider political influence, as would seem logical (since fewer voters will identify with a less mainstream set of policies)?
To what extent could it be argued that the Tea Party in the US is a Libertarian version of an entryist group in the mould of Militant?
2 This is not intended as a pejorative; if there's a better word I didn't think of, I'd be interested to hear it.