John Rentoul: One hot potato makes a mash-up

Making political capital of immigration is a hazardous ploy for the Prime Minister and the coalition

The new orthodoxy on the liberal side of politics is that immigration is an issue that must be faced, preferably squarely, and not brushed aside as a code for racism. So when the Prime Minister makes a speech about it, what do liberals do? They welcome it as a chance to face the issue squarely and to debate the substantive policy arguments made – I don't think. No, they brush aside the issue and accuse David Cameron of issuing a coded invitation to racists to vote Conservative in the forthcoming local elections.

The mistaken metaphor of the moment is the dog whistle. The whole point of a dog whistle is that only dogs can hear it. If Cameron were trying to bypass the liberal media, Vince Cable and the One Nation element of his own party base, sending a secret message to racist voters tempted by the British National Party, then it was the most unsuccessful signal since Admiral Nelson was told to withdraw.

Actually, as a piece of politics, although this is beside the point, the speech worked well. It distracted from the bad NHS story and allowed Saint Vince to pose as the defender of all that is decent and do-gooding about Twickenham while simultaneously allowing Overlord Cameron to pose as all that is wrathful and authoritarian about the Tory heartland, but who has to be restrained by the demands of coalition government. In other words, the coalition is working perfectly and there is no need for Ed Miliband at all. Job done.

But what was the speech actually about? Let us try to recover the spirit of the new liberal orthodoxy and to deal with immigration policy on its merits. An almost comical idea, I know, but let us try. As Hugo Rifkind said in The Times, it was "a good speech, but it was disguised, quite heavily, as a bad one".

It was a thoughtful and well-written bringing together of the Government's policies, in the way expected of a prime minister. Although it was deemed to be party political and was published on the Conservatives' website rather than the No 10 one, it was a speech that Gordon Brown or Tony Blair could have delivered – minus the misleading implication that the coalition had inherited a mess from Labour and that May last year marked a sharp change of policy rather than a continuation of it.

To understand immigration policy, we need a different metaphor from the dog whistle. We need to think of someone carrying too many potatoes in a wet paper sack. This is a Bangladeshi saying, according to the late Anthony Bevins, once my boss on The Independent's political staff. His wife, Mishtu, was from Bangladesh, which makes it a fitting analogy for the present purpose.

Theresa May, the Home Secretary, used it – in spirit – when she compared managing immigration to squeezing a balloon: "Push down on work visas and the number of student visas will shoot up. Clamp down on student visas and family visas will spring up." Cameron liked the balloon metaphor so much that he quoted it in his own speech. I prefer the bag of potatoes, but the principle is the same, and neither May nor Cameron seem to realise how it will continue to apply to them.

The biggest hostage to fortune in Cameron's speech was his vainglorious claim, after he has been Prime Minister for less than a year, to have solved the problem. He went through the bulges in the bag, one by one: economic migrants, family unions, student visas, illegal immigration. Curiously, he left out asylum, eventually brought under control by the previous government. Although, in a perfect illustration of the balloon or bag analogy, as soon as asylum applications came down, student visas rose – they had "almost trebled over the past decade", Cameron said. "Now, because of what we're doing, this country finally has consistent controls right across the immigration system."

That is a remarkably foolish claim, as Margaret Hodge points out in our special report on immigration today.

It was striking that Yvette Cooper, the Shadow Home Secretary, did not accuse Cameron of using immigration for electoral purposes, but chose to attack him, by implication, from the right. She suggested that his new controls would not work, saying that while he is "cutting one set of student visas, he is simply expanding another, student visitor visas, which won't count in the net migration figures". In other words, he might stop the potatoes popping out of one hole only for the bag to rip somewhere else.

The weakness of Cameron's speech was not therefore that he shouldn't have made it when he did, but that he claimed to have more control over immigration than he has. He tried to dispose of the "myth" that "we can't control immigration significantly".

On this he faces a dilemma. He wants to reassure people that the Government has a grip; yet by claiming to have got a grip he risks raising expectations that cannot be met. The Labour government also wrestled with the wet bag of potatoes, trying to control flows that were mostly driven by Britain's economic success. David Blunkett, Home Secretary when the European Union expanded in 2004, made what Cameron now regards as a mistake in allowing the free movement of Poles and Czechs, with no inkling that a million of them would come to help keep the boom going.

That is not going to happen again, and the recession means that the economic pull of immigration is weakened, meaning that it will be easier for the coalition to claim to have reduced numbers that are bound to be lower anyway.

But that won't last for long. Soon Cameron will find that the sack is splitting again, and he may come to regret claiming that he has effected a comprehensive, whole-bag solution to the problem. He thinks he's got one of those bags for life for which you pay 10p extra in Waitrose, but he doesn't realise that it isn't big enough either. The handles will soon break and the seams will split.;