Tories and the dustbin of history
The methodology in each is the same - take as your starting point 1931 (perhaps the single most atypical General Election in the entire 20th Century); note that in this election, in which the Conservatives ran as a Grand Coalition party, with the backing of the National Labour Prime Minister, the Tories managed their highest ever polling of 55%; note further that since this historical high-water mark, the Tories never managed to poll as highly again; conclude that the Tories are doomed forever. In his original analysis, Ross concluded that the Tories' inevitable destruction will herald a crisis in British capitalism.
There is one main problem with this statistical determinism - and it is what you hear mumbled away in the small print of stockbrokers: past performance is not necessarily indicative of future results. Looking solely at the statistics, an observer in 1992 might reasonably have concluded that since the 1970 election Labour had failed to win 40% of the electorate, and that its core vote had now clearly dropped to roughly 30%, Labour was doomed never again to hold power.
There is, in fact, a second problem - John Ross is about as good a predictor of political events as a Mayan calendar. In a previous life he was the lead theoretician for Socialist Action (a Trotskyist splinter that managed to get its men into practically every high-level position that the then-Mayor of London could think of). In that capacity he was not, shall we say, notable for any great gift of clairvoyance. In 1989 he confidently asserted "We are not about to witness the re-establishment of capitalism in either Poland, Hungary or Yugoslavia." In 1991 he supported the attempted Generals' coup in Moscow, and said that the ruling class had to learn "that they will be killed if they do not allow a takeover by the working class".
This isn't a disinterested statistical analysis; it's a triumph of wishful thinking.