You can’t fool all of the people all of the time, said Abraham Lincoln. Theresa May faces a different question: for how long can you string along an entire electorate? She has been a defender of the pledge to cut net migration below 100,000 – a pledge that was adopted in a different era, before the Great Migration got underway. Cameron more or less got away with this – until he didn’t. During the referendum campaign, news that net migration hit 333,000 changed the debate and embodied the point under discussion. Cameron was haunted with this pledge throughout the campaign: how he’ll have regretted not dropping it when he had the chance. But will Mrs May? I look at this in my Daily Telegraph column today.

Even if she thinks of some post-Brexit scheme to curb migration, we won’t leave the EU until 2019 and new controls will take years to have an effect. So as things stand, she’ll go into the 2020 election being the woman who failed her own immigration target for ten years in a row: first as Home Secretary and then as Prime Minister. The ONS migration projections are below: the ONS is not known for overestimating.

Andrew Green from Migration Watch (now Lord Green of Deddington), one of the most thoughtful voices in this debate, says the 100,000 target is useful for the bureaucracy. Drop it, and the system will give up even trying to control immigration: it’s the only stick that the Home Secretary has to beat border control agencies. It’s also a stick that Amber Rudd tried to drop in the first week of her job, saying only that she wanted immigration to be 'manageable'. I can see the logic behind this, but I've always been suspicious of the idea that 99k is okay and 200k is a disaster. First, overall, the most striking point about UK immigration is that it has been a stunning success - when you compare our situation to pretty much anywhere in Europe.

So surely it matters what type of immigration it is (a recent ICM polls show punters want more high-skilled immigration, and less low-skilled) and whether the immigrants integrate. What the effect is on employers (do they give up training Brits?) and if so what the government can do to mitigate. These are the real questions, right now not being asked because too many people are hiding behind a target that everyone knows won't be hit this side of the election and probably afterwards too.

As I argued in the Telegraph earlier this year, it’s manageable for the private sector: there’s no lack of food in Tescos, no lack of clothes in H&M. It’s government that can’t manage, because government feels the need to delude itself that 100,000 is going to happen. It just isn’t. A failure to prepare for that means misery for those who have to compete with migrants for resources.

Worst of all, that 100,000 pledge occupies the space where a decent immigration policy should be. There is so much to this debate – or would be, if our politicians could bring themselves to debate it. Paul Collier’s book on immigration, Exodus, shows what the debate could be like. It’s a brilliant, calm, impassioned read packed with facts and common sense. He shows how migration has a trivial effect on wages and the economy, for good or ill, but it does affect space, accommodation, house prices, public service availability, community. And given that Britain will continue with net migration running above twice the government target until the election, it’s time to have that discussion. Theresa May, having been judged by an unachievable target for so many years, is the ideal person to start that discussion. Or she could cling to this deceit of 100,000 target, string people along for another few years – and then try, as Cameron tried, to get away with it on polling day. At the start of her premiership she has the power to restart the debate on so many issues. Immigration should be one of them.

PS It turns out that I owe Damian Green an apology. I start my Telegraph column recounting a story told by Theresa May's allies: the the "tens of thousands" pledge came about when he blurted it out in an interview. A friend of his gets in touch to say the source was a widely-ignored 2006 Tory pamphlet, so it was not (as I had rudely suggested) policy making as done by Yes, Minister. Reading that pamphlet now (pdf), it says...

“ While the precise number for any year cannot be predicted at this point, we would expect it to be significantly less than current levels from the rest of the world outside the EU.

This referred to non-EU migration only, saying the expectation was the Tory policy would result in (rather than set out to achieve) "significantly less" than non-EU migration levels (the pamphlet shows non-EU inflow at 343,000 gross, 222,000 net). So where did "tens of thousands" come from? This seems to have escaped the historical record. But I'm quite prepared to believe that the phrase had nobler origins than the ones my waspish source suggested.