In some quarters, there is talk of calling Cameron's bluff. Conservative allies of German Chancellor Angela Merkel in the European Parliament say they will not pay "any price" to keep Britain in.
Running out of time?
To extract concessions before the referendum, which could come as early as next spring or autumn, Cameron will need to press hard for the attention of EU leaders distracted by a succession of crises, from Greece to Ukraine and now refugees.
Polls showing the migrant chaos in Europe giving the Leave camp an edge might strengthen his hand in getting the EU to take notice of him. But it could also reinforce European indifference to his demands.
"There is a risk the Europeans give up, think that Cameron can't deliver a Yes, so why bother to negotiate?" said one source close to the negotiators.
Officials in the EU's executive Commission and the EU Council, which groups the national governments, stress they will battle to the end to keep Britain in.
But they are concerned that painstaking efforts to craft the kind of subtle legal compromises familiar in continental and EU coalition politics could be undone by Britain's winner-takes-all culture. They stress a need to collaborate on a deal but fret that Cameron wants to show voters he has defeated his opponents.
One EU official said: "David Cameron needs a fight. I'm not sure the Europeans understand that. He needs one big thing."
Read MoreWhy European countries give more aid than others
Another said Cameron may need some drama to convince Britons, not least his own party, his negotiations had succeeded: "I would not be surprised if we need to have a crisis."
Other EU leaders, whom Cameron has irritated before with dramatic stands in Brussels that grab headlines back home, may have limited patience. A third official close to EU negotiators said: "He can't win by campaigning only in the UK ... We have to come back to consensus politics."
British officials highlight Cameron's diligent travelling to EU capitals as evidence he has learnt lessons from past showdowns. It also illustrates the complexity of reaching a deal with 27 different partners, some of whom London worries may confuse matters by seeking their own concessions from Brussels.
As all the players put on their poker faces, contradictory assessment and expectation is the rule. EU officials say they expect a "comprehensive" list of demands from Cameron next month and warn he cannot come back to ask for more. But London says his letter will offer only "a bit more detail".
For one senior official in Brussels, talks since the summer to work out a negotiating procedure have been "very smooth", "without arguments". Another source close to those discussions says they were "not straightforward", with division lurking below a surface of polite agreement.
Both sides are nervous about negotiating in the glare of publicity, making Brussels officials sympathetic to reluctance in London to reveal too much of Cameron's hand too soon.
Caution and patience will be the watchwords in Brussels.
"We don't need fights that would hurt David Cameron," one senior EU official said, insisting the aim was to help him win a referendum vote to stay. "This is about the future of Europe."