When Cameron kicked off the renegotiation at the European summit in Brussels, over dinner last week, it became clear that he will work broadly within parameters that are acceptable to his fellow leaders. British officials briefed that the government understood the EU treaties cannot be changed before a referendum.
That has been pretty obvious for a long time to anyone who understands the EU's rules on treaty change, or who talks to other EU governments, but not to all Conservative eurosceptics. As more of them come to realise that Cameron's ambitions on EU reform are in fact quite modest, they will prepare to attack him. He is reaching the stage where he can no longer appeal to both audiences.
At the European Council, there was not a lot of time to talk about the British question as EU leaders were busy with the pressing issues of the Mediterranean migrants and the worsening Greek situation. But Cameron managed to outline the broad headings of his demands for reform, repeating much of what he had already said on a recent tour of European capitals. The others listened politely and there was no substantive discussion. EU leaders asked the secretariat of the Council of Ministers to work with UK officials on the details of their proposals. Cameron hopes to clinch a final deal in December - allowing a referendum on membership to be held in 2016, probably in the autumn.
Some governments grumble that Cameron has not yet been specific on his ideas for reform. The Germans, however, think this is tactically wise. As soon as his list of demands is known, eurosceptics will attack him for a lack of ambition, while integrationists in other capitals will cry "impossible", and he will lay himself open to the prospect of failure. Cameron may say very little on specifics until shortly before the December summit.
EU leaders worry that domestic politics may blow Cameron off course. Will Conservative backbenchers push him to demand reforms that are unattainable, thereby making it hard for him to claim a successful outcome? So far he has not asked for much that his partners regard as ridiculous, except for the idea that EU migrants should be denied benefits until they have lived in the UK for four years. Will he carry the cut-and-thrust of Westminster politics - focused on victory and defeat - into EU councils, which usually work towards careful compromises that offer something for everybody?
Other leaders claim that if Cameron threatens them they are less likely to help. But some of them are already resigned to the December summit being the scene of a ferocious battle from which Cameron plans to emerge blood-stained but victorious. If Cameron wants to win the referendum, he will have to upset some Tories & admit that the EU is good for Britain.
The final deal that Cameron obtains will mix EU-wide reform with UK-specific provisions. The mechanisms for delivering change will be varied: declarations and decisions of the European Council, as well as legislation and promises to amend the EU treaties at some unspecified point in the future. Those amendments will not happen any time soon. Most leaders view re-opening the treaties as a mad idea - nobody knows where the process would end, and several countries would have to hold their own referendums.
As for substance, Cameron will probably win an accord on 'competitiveness' that covers extending the single market, negotiating more trade agreements with other parts of the world and curbing unnecessary EU red tape (in fact the Commission is already doing these). He may get something on restricting immigrants' rights to unemployment benefits, but nothing on tax credits unless the UK changes its own rules (for example, by introducing a residency qualification that applies to everyone, Britons included). He might win a treaty article promising to protect the single market, and an 'emergency brake' enabling any government to delay - but not stop - a decision that it thought damaged the market.
On the treaties' commitment to 'ever closer union', the British are unlikely to gain a full opt out, but words will be found to reassure them. The yellow card procedure could be beefed up so that national parliaments can more easily object to draft EU laws.
None of this will change the fundamentals of how the EU works. If Cameron tries to claim the contrary he will sound unconvincing. Besides, the essence of the campaign will be about whether Britain is better off in or out. Cameron likes the mantra that Britain should stay "in a reformed EU", as it helps him to keep much of his party together. But if he wants to win the referendum, he will have to upset some Tories and admit that the EU per se is good for Britain. If he tries to keep both audiences happy, he will fail.