Opinion polls

How much do polls change in the two months before an election?

In my round-up of February’s opinion poll trends, I wondered how much longer we’ll have to wait before seeing evidence of support moving back towards the two main parties. With under two months until the election, time is running out for any surge in the ratings for the Tories and Labour.

It’s something that is talked about regularly by both political commentators and academics. Some prediction-based websites factor just such a movement into their forecasts of the election result. But as yet there’s been no sign in the polls of a significant and sustained boost for either of the two big parties. Is it too late?

History suggests it isn’t. I’ve looked at how the polls moved in the two months before the last eight general elections. First of all, here’s how the Tories fared:

conpolls

Note that on the four occasions when the Conservatives were fighting an election as the government, their poll ratings went up as the election approached. Only in 2005 did the party’s rating barely change. As for 1979, the Tories were so far ahead two months before the election that they could afford to drop 7.3 points and still go on to win.

Now here’s how Labour performed in the same eight elections:

labpolls

In only one of the eight contests did Labour’s poll rating go up – and even then it didn’t win. Conversely in 1997, 2001 and 2005 the party was far enough ahead to see its rating drop and still go on to victory. Again, as with the Tories, there is only one example (1979) of Labour’s score barely changing.

Here’s how the various incarnations of the Liberal party have fared:

libpolls

It’s almost a mirror image of Labour’s scores. In seven of the eight elections, the Liberals have increased their share – the same elections at which Labour’s score went down. The one exception is 1987, when the Liberals went down and Labour went up, coincidentally by almost exactly the same number of percentage points.

Clearly polls can and do change in the two months before a general election. But what’s particularly striking is by how much they can change. There haven’t been any instances of a double-figure percentage point change, but the Lib Dems came very close in 2010 (+9.9) as did Labour in 1983 (-8.1).

My instinct is we won’t see anything near that this year. I’m happy to be proved wrong, however. It would be a rather dull campaign if the polls did not move at all between now and 7 May.

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Discussion

5 responses to ‘How much do polls change in the two months before an election?

  1. Liberals tend to increase voteshare in the campaign itself usually towards the end of it but early on it in 2010 due to debates.

    Liberals were in single figures in 1979 just like now but increased significantly and again I think they will achieve 10-11% in GE2015. Hence Labour voteshare should decrease a little.

    Con voteshare should increase to some degree as UKIP defectors return just to stop Mlliband.

    I expect Con to have lead of 5% on election night down from 7.4% in 2010.

    • The thing is, the role the Lib Dems played in 2010 and before (most credible alternative) is now being fulfilled by UKIP and the Greens (Lib Dems cannot be considered an alternative while they are sitting on the government benches – and it is up for debate whether they can still be considered credible). Therefore if history is anything to go by we should see these parties gaining support in the run-up to polling day. I’m not sure history is too helpful here though, as the old “two-and-a-half party” system is crumbling and electoral patterns are likely to have changed significantly as a result.

    • Labour fought a fairly decent campaign in 1987 (polished, professional etc.) while the SDP-Liberal Alliance was starting to fall apart. It wasn’t enough to help Labour win, of course, but I remember the consensus at the time was that Labour had done enough to ensure it remained the main opposition party and not cede any more ground to the Alliance.

  2. The 1987 election is famous for ‘Wobbly Thursday’ when a rogue Daily Telegraph poll a week before polling day showed Labour just 4% behind the Tories. Cue a mini-meltdown at Conservative Central Office and a £2 million advertising blitz in the last week of the campaign. The rest is history.

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