The Tories put a lot of planning into the launch of their New Year campaigning. Big setpiece speeches by the leader, press ads, posters with what look like an airbrushed David Cameron. The slogan ‘things can’t go on like this’  reminded me of their not so successful ‘are you thinking what we’re thinking?’ from the last election. Both will have walked straight out of focus group chattology. But trying to make Cameron sound like a caricature of a cabbie – rewind to his recent elf’n’safety speech – is not going to work. ‘Fings can’t go on like this …’ It is just one more way of saying ‘time for a change’ which is pretty much what their strategy amounts to.

But no matter how much planning you do, unless you have thought through the answers to all the obvious questions you’re going to face, then the plans can quickly come unstuck.

The Tories have been talking in vague, waffly terms about recognising marriage within the tax system for ages. They have also been avoiding real scrutiny for ages. But it was always going to come one day. You can’t just walk into power because the other lot have been there a long time. The surprise is not that they fell apart over it yesterday, but that it happened so quickly once the scrutiny began.

It happened because Labour, who do not have the cash for New Year poster and press blitzes, put together a very old-fashioned type of document based on the old-fashioned idea that if an opposition politician tells an audience of his or her intentions, then the other side are entitled to cost them.

And having produced Cameron’s first wobble of the year with such a simple exercise, Labour must now keep on pressing in the same way. It took time before the Tories’ inheritance tax plans became a liability for Cameron. But it did, because it was not thought through at the strategic level. It took time for the marriage tax pledge to become a problem. But a problem it now is, for the same reason. On that long list published by Alistair Darling yesterday, there will be more problems for the Tories to come.

Gordon Brown could be a bit of a nightmare when he was shadow chancellor, stopping his colleagues from making promises that their various vested interests wanted to hear. But George Osborne should have done the same. Because on the one hand, he and Cameron have the strategic message that they will deal with the deficit more quickly than Labour (slightly confused by their determination to be seen as wanting to spend more on the NHS, but put that to one side). On the other hand, speak to anyone in any pressure group, voluntary group, cause or campaign, and they’re all being told they will have Tory backing, that their issue is ‘an urgent priority,’ often that it is ‘the Number 1 priority,’ that ‘if it needs money we have to find it,’ or ‘it is not so much about money as political will’. But it all adds up to the same thing. Telling people with their eyes on one part of the picture that they can have what they want, not caring too much about the bigger picture that is someone else’s responsibility. Namely Cameron and Osborne.

All Alistair Darling did yesterday was to remind them there is a big picture and it is their big responsibility. That means knowing all the questions, and having the answers.

If they got such an obvious one wrong, there could be a few more such wobbles between now and polling day. .