David Cameron went into two general elections promising to reduce net migration to the tens of thousands. It was a lie. He never had any policies to achieve the target. He didn't even try. He never had any intention of reaching it.

His former sidekick, George Osborne, all but admits as much now that he has been freed from the shackles of collective responsibility and instead writes editorials in the Evening Standard. You may remember that Osborne was not just the chancellor under Cameron but also the Tory's chief tactician. And yet he now brands the target he went into two general elections under as "politically rash and economically illiterate".

What's more, no-one else around the prime minister supports it either. "None of [the Cabinet's] senior members supports the pledge in private and all would be glad to see the back of something that has caused the Conservative Party such public grief."

The reason why is simple. It would be devastating to the UK if it ever hit the target. The Office of Budget Responsibility estimates that reducing net migration to 185,000 a year (it's currently at 273,000) would force the government to borrow another £6bn a year – almost three times the negative economic effect of higher inflation or falls in productivity growth. Reducing it all the way to the tens of thousands would cost somewhere in the tens of billions. Katerina Lisenkova of Strathclyde University, estimates that hitting the target would lower GDP per person by one per cent in the long term.

But even if this were not the case, the policy makes no sense on the basis that no government can guarantee it even if it wanted to. After all, using net migration as the target means you are relying on a certain number of Brits leaving the country. The fewer leaving, the greater the reduction of immigrants you require. All of this means that the policy basically encourages the government to make the UK an unpleasant place to live.

So it is economically damaging, impossible to guarantee and incentivises the government to make the country unpleasant. No wonder Theresa May chose to keep it alive in her manifesto this week.

As soon as the document was released, it started falling apart. A tragi-comic interview with defence secretary Michael Fallon on Newsnight last night saw him claim that it was not a policy but an "aim" or an "ambition". And of course the policy was completely without economic justification or elaboration. Fallon was unable to say how much it would cost the Treasury if he achieved it. This is Mickey Mouse economics, if Mickey Mouse was a sado-masochist.

It's quite clear from Fallon's answers that May's administration is doing the same thing as Cameron: publicising the aim without doing anything to achieve it. After all, the raise in financial charges on firms hiring foreign workers and plans to up the income benchmark for family visas won't achieve it. Although, to be fair, the economic catastrophe of a no-deal Brexit might. So perhaps there is a cunning plan behind all this.

However, there is one distinction between Cameron and May in this respect. Those who know her suggest she really does believe in this stuff. After all, this is the home secretary of the Go Home van. That therefore makes this a dangerous moment. As things stand, she doesn't appear to be foolish enough to try to hit the target. But if media pressure for her to do so grows, she might actually try to abide by her manifesto. And that would be far more damaging to this country that the breach of trust she has created by foolishly making this promise again.

Ian Dunt is the editor of Politics.co.uk. His book – Brexit: What The Hell Happens Now? – is available now from Canbury Press.

The opinions in politics.co.uk's Comment and Analysis section are those of the author and are no reflection of the views of the website or its owners.