The interesting thing about the health "debacle", as Matthew D'Ancona described it on Newsnight last night, is "why now?"

Many cite the fact that the Liberal Democrat conference voted overwhelmingly against the plans three weeks ago. But that hardly spells ruin - nobody has taken much notice of the Lib Dem conference before.

After all, the same Liberal Democrat activists voted against Michael Gove's free schools policy last September, but that hardly imperilled his plans.

Too many people are analysing this problem as a straight Conservative-Liberal Democrat divide, but it is a lot more complicated than that.

Liberal Democrat MPs, in fact, voted en masse for the Health and Social Care Bill at second reading in January. Not one Lib Dem voted against, though two members, John Pugh and Andrew George, abstained.

The fact is there are now huge doubts within both parties, and there has been a simmering nervousness within ministerial ranks ever since last summer.

David Cameron himself ordered Oliver Letwin and Danny Alexander to stress test the legislation last autumn. More recently George Osborne has had serious doubts. But it has taken a while for those doubts to surface, partly because many politicians simply were not familiar with what Health Secretary Andrew Lansley was doing.

One minister happily admitted to me he didn't "understand" the changes.

The forestry U-turn naturally made government strategists look around and ask what else there was which could suddenly bite them.

Government whips did a brilliant job on Monday afternoon disguising the level of scepticism among Conservative backbenchers.

The most powerful Tory critic Sarah Wollaston was absent - as I reported in an earlier entry - while David Ruffley, a right-leaning free-marketeer, voiced far stronger criticism in my package on Newsnight than anything expressed by the 20 or so Tories who asked questions in the chamber.

I'm pretty confident, though, that even without this two month pause Mr Lansley's bill would have got through its remaining stages in the Commons and Lords.

The political fears were more long term. Would it have led to the closure of district general hospitals, causing the "Kidderminster effect", as David Ruffley describes it, which saw a Labour MP lose his seat in 2001 over the loss of a local hospital?

Will Mr Lansley's measures cause a crisis in the NHS around 2013 and 2014, just in time for the next election? Mr Lansley's argument to Cabinet colleagues is that there'll be a crisis in the NHS if he doesn't make these changes.

One senior Downing Street adviser describes this as a bill "with no friends", and that's a dangerous position to be in politically.

Astute reformist politicians, from Abraham Lincoln to Tony Blair, understood the importance of preparing public opinion before making revolutionary changes.

This two-month pause is being presented as a listening exercise, but in reality ministers intend it to be the public and the interest groups who do the listening, as much as them.

And there must be huge doubts whether we will get the kind of "substantial" changes Nick Clegg spoke of yesterday (and on which he seems to be backtracking today).

Another of Mr Cameron's close advisers told me: "This is all about pausing to regroup in order to advance, rather than any kind of retreat. That's the feeling in our party."

On reflection, Oliver Letwin may not have been the best person to carry out such a stress-test of Andrew Lansley's NHS plans. Reports from 2005 suggest that he originally thought Lansley should become Conservative leader.

