The truth is, as ever, rather more complex than that. Firstly we should set aside the ICM poll apparently showing a 4 point drop in Tory support as the NHS row continues. This is a classic case of making the outlier the story, the media’s constant failing in reporting polls – the underlying trend in polling is ignored while the rogue poll (and the reversion to the mean afterwards) gets a headline. Looking at the broad range of polls the Conservatives have not lost four points of support: they have gone from a position in December when, after the European veto, they appeared to be slightly ahead of Labour in the polls to a position this month when they appear to be a point behind or so, a drop of one or two points.

Many commentators today have also fallen victim to a fallacy I often see in the comments here – ascribing whatever movement happens in the poll to whatever subject they personally feel strongly about. Hence the Guardian and many bloggers feel strongly about the NHS, there has been a slight drift downwards in Tory support, therefore the former is probably the cause of the latter (it’s also a sort of availability bias – the poll contained questions about the NHS, they are bad, therefore that’s the cause. What if the other questions in the poll had been about, say, crime?)

However, this ignores other possible explanations, which could actually be better evidenced. Here are a couple, though I certainly wouldn’t claim these are exhaustive – there are no doubt other possibilities, including those that are less well served with tracker polls.

The first hypothesis is the unwinding of the European veto effect. You will remember that in November Labour had a lead of four or five points. The Conservatives then pulled level after David Cameron’s veto at the European summit. We should be expecting the effect of the veto to gradually unwind and, indeed, there is evidence to suggest that is what we are seeing.

The proportion of people thinking that Europe was an important issue facing the country peaked at 38% just after the veto, but has since faded away again, and is now back down to 23%. Similarly we saw some sharp increases in perceptions of David Cameron on the back of his veto. The percentage of people thinking Cameron “sticks to what he believes in” rose from 26% to 39%, decisive went from 20% to 29%, good in a crisis went from 13% to 18%. Since then most of these figures have fallen back down a bit (“stick to what he believes in” back to 31%, decisive back to 24%, good in crisis back to 18%).

One straightforward explanation therefore is just the fading of the leadership boost that Cameron received from the veto.

A second hypothesis is less negative coverage of Ed Miliband. In January the media narrative was dominated by criticism of Ed Miliband’s leadership, and the media impression was increasingly that he was a no hoper. In February that has faded a bit, the attacks have mostly stopped coming, he’s had a few good PMQs under his belt and the media narrative has largely moved on (to a great degree to the NHS!). This is reflected in Miliband’s own ratings. In January he hit record lows in his approval ratings, reaching a nadir of minus 53 points. In February he has recovered a but to the low minus 40s. His ratings are still significantly worse than they were last year… but he has risen off the canvas.

A third hypothesis is the NHS. The recent coverage has certainly pushed the NHS up the agenda – going back to the YouGov issues tracker, the proportion of people naming the NHS as an important issue is up to 32% from 22% a month ago. We also have lots of questions that have shown the government’s policy is unpopular and that people don’t trust the Conservatives on the NHS, but these don’t necessarily indicate change – it could’ve been unpopular all along.

The ICM poll yesterday shows that people trust the Conservatives less on the NHS now than in 2006, which is an interesting finding in itself, but it doesn’t follow that the drop in trust has come in the last few weeks or is a result of the current policy. If we look at YouGov’s regular tracker on which party people trust the most on the NHS there is a distinct lack of any recent drop in the proportion of people who prefer the Conservatives on the NHS.

In fact, the Conservative trend in the last year is pretty flat – what drop there was happened in late 2010 as the government’s honeymoon faded. What is also worth noting in this graph is that the Conservative detoxification on the NHS is something of a myth – it never really worked very well, and Conservative leads on the NHS were small and transitory.

This, I suspect, is one of the reasons the NHS hasn’t damaged the Conservatives that much – most people never trusted them on it in the first place. The other reason is that most people have no clue about the reforms and what they are or are not likely to do – take this recent YouGov poll for the Sunday Times which, rather than tell people about the reforms and ask about them, just asked people to say if they supported or opposed them based on what they’d seen or heard about them. 48% of people were opposed… but over a third (34%) of people said don’t know, a comparatively high figure for any question.

That is not to say that the NHS reforms are not politically dangerous for the government – I think Tim Montgomerie’s point that any future problems with the NHS (and there will always be some problems with the NHS, even if there were not budget squeezes and reorganisation) will be blamed on the policy is well made. Equally, just because it hasn’t made a big impact so far doesn’t mean it couldn’t do so in the future.

However the claim that it is already doing significant damage the Conservatives in the polls is weak. The drop in Conservative support in the polls is small and there are alternative and perhaps better evidenced explanations for it.

There is also an interesting question on trust in the NHS, repeated from 2006 early in David Cameron’s leadership. Back then 14% said they trusted the Tories a lot on the NHS, 47% trusted them a little and 31% didn’t trust them at all. The figures now at 13% who trust the Tories a lot (down 1), 42% who trust them a little (down 5), 40% do not trust them at all (up 9). This suggests a drop in the proportion of people who trust the Tories a little on the NHS, though the hardcore of people who trusted them a lot was minimal to begin with.

On the same question for Labour 23% trust them a lot(up 4), 46% trust them a little (up 2), 25% do not trust them at all (down 7).

We’ve certainly got the daily YouGov poll for the Sun to come tonight, I don’t know if Populus and Ipsos MORI’s monthly polls will also turn up today or tomorrow, or whether they’ll be done next weekend.

UPDATE: The monthly Populus poll for the Times has now been released, topline figures are CON 37%(nc), LAB 39%(+1), LDEM 11%(-2). Putting aside the usual variation we get between different companies Liberal Democrat scores we are actually getting quite a consistent picture on the Con vs Lab horse race – YouGov’s daily polls have been averaging a Labour lead of about 1.5 points, ICM tonight are showing a 1 point Labour lead and the Populus are showing a 2 point Labour lead.

UPDATE2: And finally (assuming MORI don’t pop out of the woodwork), the Sun politics team have tweeted the YouGov figures for tonight. Topline figures are CON 39%, LAB 38%, LDEM 10%, so while the lead is bouncing back and forth (much as we’d expect from normal variation within the margin of error), it’s still very much in line with YouGov’s average position of a small Labour lead (and the 7 point figure for the Lib Dems yesterday does appear to have been an outlier).

1) The problems of the new child benefit rules. The principle of withdrawing child benefit from households with a higher rate taxpayer remain very popular, with 64% of people supporting it and only 25% opposed. However, the practicalities whereby a household with two earners paying basic rate could have a higher income than a single higher rate taxpayer, yet still get child benefit, is still seen as unfair by 68% of people.

2) Perceptions of coalition. In principle 22% of people now think that coalitions are a better form of government than single party government, 55% think single party government is better. 21% of people say that the experience of the current coalition government has made them more positive about coalition, 39% more negative (ideally of course there would a “attitudes towards the principle of coalition” question from before the last election to compare things to, but alas, I couldn’t track one down)

3) Selective education. 37% of people would like to see more schools select by academic ability, 20% are happy with existing grammar schools to stay but oppose any expansion, 27% think the existing grammar schools should be opened to children of all abilities. There was a surprisingly positive response to the idea of re-introducing the assisted places scheme, suppored by 67% of people.

4) Religion. 12% of people think religion is more often the cause of good in the world compared to 58% of people who think it is more often the cause of evil. Conversely, only 17% think Britain is too religious while 36% think it is too secular (the apparantly paradox is probably a difference between thinking globally, and the thoughts of terrorism and armed conflict linked to religion that would almost certainly have come to some people’s minds, and thinking locally)

Normal caveats apply, it could the start of a trend, or it could just be normal margin of error – we’ve had one 4 point and one 5 point Lab lead this month, and a poll last month showing the Lib Dems at seven, and in both cases things were back to rather more normal figures the next day.

That said, while YouGov’s daily polls are flitting between Labour and Conservative leads due to normal variation within the margin of error, we are seeing rather more Labour leads than Tory ones, and rather bigger Labour leads than Tory ones, suggesting the underlying position is a small Labour lead of a point or so (the average Labour lead in YouGov’s polls so far this month is 1.5 points).

Topline figures, with changes from ComRes’s last online poll a month ago, are CON 39%(+1), LAB 38%(nc), LDEM 10%(-1), Others 13%(nc). The changes from the previous ComRes poll are insignificant, but for the record it’s the first time ComRes’s online polls have shown a Tory lead since October 2010 and supports the conclusion that the polls are pretty neck and neck.

UPDATE1: Hold the presses. ComRes’s press release said the Conservatives were one point ahead, but the actual tables are here and it looks to me as if Labour are one point ahead (pages 19-21 of the pdf). Let me double check.

UPDATE2: Waiting for Andrew Hawkins to have a check of the tabs

UPDATE3: According to Andrew the topline figures were right, the tabs were wrong, so back to business as usual. Corrected versions of the tables should be up soon if not already there.