The evidence is mixed
The above chart was compiled by taking the polling average closest to the 18 month point before election day.
As can be seen the first opposition leader to break the rule was Mrs. Thatcher in 1979. Jim Callaghan’s government performed worse on election day than the polls 18 months beforehand – but then, of course, there was the “winter of discontent”.
In 1983 Mrs. Thatcher saw the biggest variation on the polling average 18 months earlier but then, of course, there was the Falklands War.
1987 saw a significant recovery while in 1992 I have taken the polling figures from the first month of John Major’s premiership.
1997 might have been disastrous elections for the Tories but John Major’s party performed substantially better in vote terms than the polling a year and a half out.
In 2001 the incumbent Blair government did substantially worse than the polls in late 1999 were suggesting.
In 2005 and 2010 the incumbent governments did perform better but not by very much.
Some of this, certainly in the early years, is down to poorer polling methodologies than we have now. Interestingly the last two elections when most pollsters have embraced measures to ensure politically balanced samples, the variation has been much smaller.
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