What happened when Major replaced Maggie
The table which I have adapted from the ICM web-site shows what happened to the party ratings in the ICM Guardian polls before and after John Major replaced Margaret Thatcher in November 1990.
Polling, of course, was very different in those days and nothing like as accurate as it is today. Major went on to win the 1992 general election with an 8% lead on the national vote – something that none of the pollsters predicted.
Following that debacle pollsters like ICM introduced new methods to deal with the systemic tendency for polls to over-state Labour.