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alternatehistory.com
The site www.electoralcalculus.co.uk allows you to estimate the effect on historical UK elections on changes in each party's vote share, which is a great alternative history tool. Their database goes back to 1983.
Starting in 1983, I am calculating the effect on each election on small changes in vote share. My rule is to change the vote share of one of the major parties party by 3%, the second by 2%, and the third by 1%.
The point of these small changes is that they could easily result from minor changes in the events of the campaigns. Then butterflies result.
For 1983, I decreased Labour's share of the vote by 3%, to 25%. I bumped the Alliance up by 2%, and the Tories by 1%. This means the Alliance overtaking Labour in the popular vote.
What would have been the effect of this?
See the results here, though I will try to embed them in the next post: