For those willing to engage their brains rather than their prejudices, polls are a useful tool when properly used. Not all polls are the same. The way they are conducted varies. I have spent many hours discussing with Populus, pollster for The Times, how to ensure that our monthly surveys are as representative as possible. (Details of methodology appear on the Populus website.)

Differences over weighting for past votes and for “don’t knows” account for some of the apparent contrasts in this week’s polls. There are some good rules: always look at levels of support, not leads, which exaggerate changes; disregard variations of one or two points, which are within the margin of error; compare the results from the same polling organisations, not different ones; and look at underlying trends. (Anthony Wells, of UK Polling Report, and Mike Smithson, of Political Betting, offer informed web commentaries on these issues and on polls.)

With these caveats, the apparent variations are smaller and polls are easier to understand. Conservative support has fluctuated in a range of 35 to 38 per cent since the early summer, sometimes drifting to the bottom of the range, at other times to the top. Yesterday’s ICM poll in The Guardian put the Tories on 39 per cent, a rise of one point compared with an ICM poll in early October. It is possible, but no more at present, that the Tories are moving up to a new level. While some analysts believe the Tories benefit when they are in the news, this was not true after their party conference. And last week’s message on tax was confused.

The implications are more worrying for Labour. Its support has been in the 31 to 33 per cent range over the summer, moving up earlier this month, but apparently falling back now. The ICM figure of 29 per cent, down three points on early October, is outside this range — though it could be a blip, not a new trend.

The most plausible explanation is that the public debate over the continued mess in Iraq and the withdrawal of troops may have reinforced doubts over Mr Blair and Labour. In the past, any mention of Iraq has boosted Liberal Democrat support, which has risen two points to 22 per cent in the new ICM poll. All eyes will now be on the YouGov poll due this week, and the next Populus poll in The Times in 12 days’ time. Remember, even on the ICM figures, the Tories would, in theory, still be just short of an overall Commons majority, though they might be helped by a reversal of past tactical voting. At least the Tory vote is firming, while Labour support appears soft. If this carries on, expect more calls for an early change at the top.

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