The Conservatives now have a consistent lead in the opinion polls. ICM, Populus and YouGov have all shown a Conservative lead since back in April. The response to each poll from the Labour party (and from more sage and cautious Conservatives) is to wonder whether the Conservatives have achieved the sort of lead that a party needs at this stage to go on to win an election.

Questions like this always rankle with me to an extent becuase there is a danger of taking a too deterministic view of politics, so first let me get this off my chest: obviously there is always the possibility of “events, dear boy”. A government could have stonking great leads in the polls throughout a Parliament, and then get caught collectively shooting kittens on the eve of election night and lose. Obviously things like wars, mass strikes, economic disaster and so on can change the whole picture. Assuming none of these things happen though, what sort of lead should a party on its way to a victory be enjoying?

Throughout the 1980s the Labour party used to stack up huge double figure leads…and yet lost. Before the eventual Labour party victory in 1997 the party had enjoyed polling leads of over 40 points at some points. The single figure Conservative leads we see today look anaemic in comparison; if they are going to stand a chance of winning, surely they would be in a better position than this?

Perhaps not. Firstly the pollsters themselves have changed since the 1990s, as have the methods they use. The 40 point leads that Labour enjoyed were reported by Gallup, who no longer produce political polls in the UK. MORI produced 30 point leads for Labour in the 1992-1997 Parliament, but that was before their polls prompted by party name or accounted for people’s likelihood of actually voting. Neither YouGov nor Populus were around back then, in fact the only vaguely comparable figures from the 1992-1997 Parliament are ICM’s, who had already learnt the lessons of the 1992 election and adapted their methodology (and it’s worth noting than while ICM adapted their methods after 1992, they didn’t switch to phone polling until 1995).

While MORI were showing double digit Labour leads in 1993 and Gallup were showing leads of over 20 points, ICM was producing figures showing Labour around 5, 6, 7 points ahead (overall ICM’s Labour lead that year ranged from 14 points ahead to a 2 point deficit). In 1994 Labour moved further ahead, but ICM were still showing leads in the mid-teens range, as opposed to Gallup and MORI showing leads in the 20s and 30s. With modern polling methods the days of the opposition leading the government by 30-odd percentage points are probably history. A reasonable yardstick to measure the Conservative opposition’s performance against should be the ICM figures from 1992-1997, and while they have recently started to fall short of the double-digit leads Labour started to record in 1994, they are certainly still in the same sort of ball-park as Labour’s 1993/4 performance.

So, when people say that, up against a government on its way out the Conservatives should be doing X amount better at this stage in the Parliament they are playing politics. Go back a decade or two and polling techniques were different, they aren’t comparable and if you look at ICM’s figures in 1993/4 they show similar pictures. Unfortunately for the Conservatives, this certainly doesn’t mean they are doing well enough to win the nextelection.

At the beginning of 1994 ICM showed Labour with a 12 point lead over the Conservatives. Three years later they went on to win with a lead of 13 points, giving them a majority of 177. An equivalent Conservative lead would give them a meagre 34 seat majority. Back in 1997 Labour could have got an overall majority with a lead of about 2 points over the Tories on a uniform swing, so their poll leads of 6 or 7 percent in 1993 were actually enough to win an election. These days, again assuming a uniform swing, the Conservatives would need a lead of around eleven points to get an overall majority.

In practice I expect the Conservatives would gain a majority on a slighter lower lead as a result of the shifts in the pattern of votes and tactical voting that such a large change in public opinion would imply. The fact remains however that the Conservatives need a much bigger lead in the polls to win an election than Labour do (the reasons for the apparant “bias” in the electoral system are explained here). The Tories aren’t doing much worse than Labour were at a similar period in 1993/4, and haven’t had Black Wednesday to help them. The difference is that in 1993 a six or seven point lead was easily enough support for Labour to win an election even if they had ended up losing a point or two before polling day. For the Conservatives today, even if they maintain their present support, it’s unlikely to be enough to secure a majority.

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