Implausible and vote-winning – is the PM’s migrant cap plan designed to fail?

Downing Street's immigration crackdown, dismissed by critics as a blatant bid to out-Ukip Nigel Farage, is unlikely to become reality. That may not worry David Cameron very much.

Jose Manuel Barroso, the outgoing president of the European Commission, has been obliged to equivocate once or twice in his decade running the EU's civil service. But today his rejection of the prime minister's looming U-turn on immigration could not have been more absolute.

"Any kind of arbitrary cap seems to me to be not in conformity with European rules," he said. "The principle of freedom of movement is essential, we have to keep it."

His comments on BBC1's The Andrew Marr Show came after the Sunday Times revealed details of the 'emergency brake' Cameron is preparing to unveil in order to woo voters in the upcoming Rochester and Strood by-election.

The big idea emerging from No 10 is a cap on the number of migrants from the EU. This wouldn't stop people travelling to and from Britain. Instead it would prevent more than a certain number of low-skilled workers receiving national insurance numbers every year – allowing access to the system that gives workers access to certain benefits, including the state pension. While everyone would receive a number initially, the 'emergency brake' would involve the national insurance number being removed after a period of time.

A source told the paper:

"There would be a quota system to have time-limited national insurance numbers. The prime minister is now in the place where he wants to reduce the numbers coming in, not just the pull factors."

This is a big deal because it represents a substantial U-turn from Cameron. It also sets him up to fail, as the proposal appears likely to fall foul of the basic principle of freedom of movement which forms such an important pillar of the EU's internal market.

Barroso, who is not quite yet in demob-happy mode as he prepares to end his time in the job, could barely keep the sarcasm out of his voice as he effectively told Cameron: 'Good luck with that.' What he actually said was:

"Prime Minister Cameron is free to propose whatever he wants with colleagues, to colleagues, because we need unanimity to change basic rules in the European Union. My advice to all the members, including Britain, is to have a constructive dialogue, to have the proper tone, not speak about weapons of fire between partners."

Barroso also provided some clues as to the kinds of hostile arguments Cameron would face if he were to take this idea to Brussels. The Commission's officials would point out that Britain has kicked up a stink over freedom of movement relating to Gibraltar and Spain. They'd put the 1.4 million Brits living in other EU countries in play. And they'd try and placate Britain by offering concessions on abuses of the benefits system. What they would not do, he made clear, is back down.

The Conservatives are not listening. They seem convinced this new plan could work. At the back of their minds there may be an awareness that, whether it's doomed to failure or not, it may at least help the Tories beat Mark Reckless in Rochester and Strood.  They have certainly not yet come up with any compelling reason why No 10's U-turn should succeed. Justine Greening, the international development secretary, told Sky News' Murnaghan programme freedom of movement was never "supposed to be a totally unqualified principle irrespective of how it works on the ground". Lord Heseltine took refuge in the idea that this could only apply to new arrivals in the EU:

"It's been negotiable ever since we've had new accession countries. There's always been an anxiety that opening the door would produce a huge number of immigrants in a very short period of time. What David Cameron is talking about is not saying we're going to have a totally different shutter type of approach – there's just a limit that a host economy can absorb."

It doesn't seem to make any sense. Cutting back on immigration is politically attractive but practically implausible. Even if it doesn't get scuppered by officials in Europe, the policy's ultimate goal – finally achieving the Tory target of cutting immigration below 100,000 – seems to be as dogmatic as it is vulnerable. The unsuccessful pursuit of the target in this parliament has hit higher education hard, as overseas students were easy meat for desperate Tory ministers.

Few people talk about the cost to the economy of these steps – and the party whose job it is to oppose the government doesn't seem very interested in doing so."Labour is in favour of reform to European free movement rules and we will examine any proposals the government comes forward with to manage immigration with interest," shadow immigration minister David Hanson says.

The opposition wants to "stop people travelling to claim benefits, deport people who commit crimes and stop employers undercutting local jobs and wages with cheap migrant labour", he adds. Labour fears the Ukip threat in the north of England almost as much as the Tories do – making this oddly misshapen debate about immigration depressingly one-sided.

Cameron may have a longer-term plan in mind. Taking on a tough, apparently unwinnable fight like this follows a logic the PM has already discovered helps his cause when dealing with troublesome backbenchers and the Ukip threat they reflect. As he first learned with the treaty veto, the Cameron setpiece process looks something like this:

– Drastically overbid in negotiations with Brussels
– Try hard but ultimately accept defeat
– Accept political kudos from impressed eurosceptics
– Incorporate failure into case for in-out referendum

The prime minister's European policy is based on deferring the big European reckoning until 2017. As with the Scottish independence referendum, the assumption is that with the stakes piled high voters will make the right decision when forced to make a choice. It's a risky strategy – but keeps the most challenging political problems in a manageable box until then.

The immigrant cap plan slowly emerging from Downing Street forms part of this. As Barroso's contempt shows, it is unlikely to succeed. Anticipating defeat, the PM can always fall back on the consolation that this will help the Tories' chances at the next general election – a contest the Conservatives must win for Cameron to even have a chance of holding the 2017 vote. Yes, this will make winning an in-out referendum harder. But that is not the priority right now.

This long-term strategy explains Cameron's thinking. It makes the incomprehensible reasonable.

The PM is pursuing an idea as popular as it is nonsensical. One, it seems, holds the key to the other.