FOR months David Cameron has refused to set out exactly what he wants from his renegotiation with the European Union before his in/out referendum. That is because whatever he asks for will instantly be denounced as inadequate by the prime minister’s own Eurosceptic backbenchers. Yet at the European summit on October 15th-16th he was forced by irritated fellow leaders to promise to put his demands in writing early next month. And, as he repeated this week, high up his wishlist is a determination to exempt Britain from the treaty commitment to “ever closer union”.

The full formulation is an “ever closer union among the peoples of Europe”, a subtle but important addition. This phrase occurs in the preamble to the 1957 Treaty of Rome and in most later treaties. Yet until recently even Eurosceptics did not object to what is merely an aspiration. Some other governments have expressed scepticism about the goal. In 2013 the Dutch government declared that “the time of ‘ever closer union’ in every possible policy area is behind us”. And in June 2014 the European Council formally said that the concept embraced different paths of integration for different countries, “allowing those that want to deepen integration to move ahead, while respecting the wish of those who do not want to deepen any further.” So why is Mr Cameron using scarce negotiating capital to scrap the provision for Britain?

The short answer is that he needs a gesture to appease his Eurosceptics, who have not had a good week. On October 20th the Confederation of British Industry, the main business lobby, claimed that “the majority of British firms believe that the pros of EU membership outweigh the cons”. Philip Hammond, the foreign secretary, also called it “completely unrealistic” to seek an individual national veto. A day later the governor of the Bank of England, Mark Carney, declared baldly that EU membership had made the British economy stronger and more dynamic, adding that Britain had been the leading beneficiary of the EU’s four freedoms of movement of goods, services, capital and people.

Ever closer union clearly has political resonance, even so. Mr Cameron claims that, when Britons decided in the 1975 referendum to stay in the European project, they saw it as a common market, not a political union. Yet that claim seems strange given the preamble. Moreover, as Alan Johnson, chairman of Labour’s European campaign, recalled in a speech at the Chatham House think-tank on October 20th, “all the debate on both sides in 1975 was about political union”.

Even more important, the phrase may have some legal weight. Jean-Claude Piris, a former chief legal adviser to the EU council, stresses its application to peoples, not states, and says the formula is too vague to have any legal force. But the European Court of Justice (ECJ) has quoted it several times. A recent study by the House of Commons library finds references to ever closer union in 53 ECJ judgments. Martin Howe, a British barrister who specialises in EU law, says the preamble “strongly influences the ECJ’s interpretation of other treaty articles and general principles of EU law”. One example was a ruling last December against the right of the EU to accede to the European Convention on Human Rights, which cited the goal of ever closer union.

The problem that this creates for Mr Cameron arises because the most he can expect from his partners is a protocol enshrining the European Council’s June 2014 conclusions about different paths for different countries. There is no chance of 27 other EU countries ratifying a treaty change that drops the preamble altogether. So the ECJ may go on citing it whatever Mr Cameron wins, making any British exemption less valuable.

There are two ironies in all this. First, the preamble was kept in the 1992 Maastricht treaty partly at British request, to forestall an alternative reference to a federal union. John Major, then Tory prime minister, was even praised for adding a rider about decisions being taken “as closely as possible to the citizen”. And second, a decade later, the draft EU constitutional treaty ditched the offending phrase altogether. Yet Mr Cameron’s Tories were among the loudest critics of that treaty—and, when it was rejected by French and Dutch voters, ever closer union quietly reappeared in the 2009 Lisbon treaty.