David Cameron’s pledge to use an ‘emergency brake’ to cut EU migration to the UK seems to be an empty threat, a sign of mere posturing rather than a well-researched policy initiative, writes chief political correspondent Tim McNamara.

He promises “one last go” in order to obtain “more effective control of migration”, yet fails to offer any specific action he would take if his one last go actually goes nowhere. He now promises detailed proposals before Christmas. Could he take the nuclear option and put in controls over EU migration?

Whilst sympathetic to Cameron’s political problem, the outgoing European Commission president, Jose Manuel Barosso, repeated his view in a speech at the think tank Chatham House, that an “arbitrary cap” on immigration would not accepted by other EU states, saying the Commission was willing to “accommodate the UK’s legitimate concerns” but not if they challenged the fundamental principles of EU law.

Mr Barroso said the UK had “many friends” in the EU. He acknowledged that there were “widespread concerns in the UK and elsewhere about abuse of free movement rights”.

But he added: “Changes to these rules need all countries to agree. And it is an illusion to believe that space for dialogue can be created if the tone and substance of the arguments you put forward question the very principle at stake and offend fellow member states.”

But what if Cameron’s threat actually has substance? Would selective counter-measures against the free movement of persons in the EU be possible even if they were not efficacious? Could Cameron implement a policy that, in the short-term, be both electorally advantageous and popular in some parts of the UK? Even if the measures were found to be illegal at a later date? What if the lifetime of such measures were to straddle the upcoming election campaign in the UK (with the election due in May 2015)? They could have short-term political benefit which would have to balanced against long-term consequences.

Would the Home Secretary, Theresa May, be able to implement such a government initiative without the support of the Tories’ coalition partners, the Liberal Democrats? Would such a precedent be expeditiously open to challenge in the British courts? What if the Prime Minister invoked the ‘The Luxembourg compromise’?

This was a solution to the French boycotting meetings when (in their view) their vital national interest had not been taken into account. However, this was a different time with different politics; France had most to lose and Germany and Italy called it‘s bluff.

The UK has a long and proud history of implementing (and abiding by) EU legislation. It is sometimes accused of being too enthusiastic in enacting EU Directives, with civil servants accused of ‘gold-plating’ Directives so as to add extra policy decisions to the legislation in order for Ministries to get their pet projects through Parliament.

Would the opportunity to shoot the UKIP fox be irresistible if border controls were seen to be in introduced some time before the general election in the UK?Such a decision is more likely if Cameron’s shock troops fail to stop UKIP winning the UK’s next by-election in Rochester & Stroud, Kent on 20 November.

It is likely that Cameron will stick to the line ‘that something will be done’ without specifying what that ‘something’ will be before the by-election. However, if he knows that the predicted result is that there will be two UKIP MEPs in the House of Commons, he may well act sooner. Cameron may well be panicked into announcing some measures that will imply he has to do something to stop any more defections from the Tories to UKIP.

The European Commission could make a decision quite quickly but the UK could ignore it and it would take a while for the ECJ to declare measures were illegal and then the UK would have to back down. There have been other examples of short-term measures responding to raw national politics trumping EU law before.

As a New York Times report stated “All the crisis has done is to stir up anti-British feelings across the Channel and xenophobic tendencies at home. It put on display the deep divisions here over integration with the rest of Europe.” However that was in article of June 1996 covering John Major’s policy of non-cooperation over the (quite reasonable) ban on British beef during the ‘ Mad Cow’ crisis. Would Cameron be prepared to go further?

The timing of such a blatant disregard of EU law may be fortunate. The new European Commission may not be as bold in its early days as the previous one. The new president, Jean-Claude Juncker, has intimated that he has some sympathy with Cameron’s predicament in the UK. Nobody understands a politician’s electoral difficulties like another politician.

The policy could not necessarily take a broad-brush approach; it could not apply to all parts of the EU for it to be practical and not risk significant retaliation by other EU member states (such as Spain and France). There’s more to lose than to gain with several Member States. There are also large numbers of workers from countries such as Poland already living and working in the UK for many years.

However, action selectively targeted against Romania, Bulgaria and Croatia, may be possible to introduce without a huge political backlash either in the UK or may other parts of the EU. The introduction of an Australian-style ‘points system’ for these three countries may be very tempting for the Tories and give their activists something to challenge UKIP on voters’ doorsteps.

In 2010 France took direct action against some Romany immigrants from Romania and deported them. The then EU Justice Commissioner Viviane Reding described the deportations as a “disgrace” and the European Commission took a first step towards legal action against France.

But, the Commission refrained from opening a case against France for alleged discrimination, instead demanding more proof to support France’s claim that it was not deliberately targeting Roma. Wholesale action against an ethnic minority would have violated EU anti-discrimination laws, including the EU’s Charter of Fundamental rights.

The French policy made huge headlines in France and across the EU. Yet the measures were short-term, domestically popular and avoided an infringement procedure from the European Commission. Interestingly, the possible infringement procedure was to concentrate on racial grounds and not on the deportation of a set of EU citizens to another EU country.

The UK could argue that it was not discriminating against any race if any measures applied to all potential immigrants from Bulgaria, Romania and Croatia. Croatia already has strict transitional controls on immigrants into Croatia from other EU countries.

With immigration seen as the biggest concern in many marginal seats, the headline is important – the devil is in the detail.  An initiative launched in March could have a decisive impact at the May general election.

It would have the desired effect of getting most people in marginal seats talking about immigration and drag the political conversation into an area Labour and the Liberal Democrats still feel awkward about. It may, though, play into UKIP’s hands, if it is overplayed.

Cameron seems to be being forced into a corner. He may well respect the rule of law in the EU but domestic pressures seem to drive ever more in a Euro-sceptic direction. Like blackmailers, the Euro-sceptics keep asking for more no matter how much he tries to buy them off.

It may be viewed as a cynical political calculation but there is nobody more calculating than a political leader desperate for an electoral advantage in a tight race. Would it be seen as a price worth paying for increasing chances of electoral success?