Ed Miliband challenged David Cameron in parliament today  on his negotiating position ahead of Friday's European Council summit. The charge was that the prime minister has promised something to his backbenchers that he cannot deliver - a repatriation of powers from Brussels. It was a line calculated to probe Tory eurosceptics' anguish over their leader's failure to capitalise on the opportunity (as they see it) of the eurozone crisis to bring about the longed for renegotiation of the UK's relationship with Brussels.
That was a fairly easy hit for a leader of the opposition. Why, when the prime minister's biggest political bruise is exposed, would he do anything other than punch it? At the moment, Labour doesn't really need to do much on Europe other than find new ways to exploit coalition pain on the subject. And just to be clear, there is a lot of pain out there. Cameron is in an appalling position. He has to go to Brussels and somehow persuade fellow EU leaders that he wholeheartedly endorses their plans to save the euro with a treaty for much closer integration, while pointing out that his party thinks a treaty for much closer integration is an affront to democracy and human dignity, so could he please have a bunch of concessions on issues unrelated to the euro, otherwise he might have to veto the whole thing. If he fails to pull that off, his backbenchers will feel betrayed. And if he manages to get concessions, they probably won't be big enough and his backbenchers will demand a referendum on the new treaty. They can sabotage it in parliament if they don't get one.
It all adds up to a Christmas hamper of opportunities for Labour. But if, as is quite possible, the situation turns still more critical for the single currency and David Cameron, Ed Miliband will also have to start formulating a position on his preferred outcome. At the moment, Labour's EU policy has been spelled out by shadow foreign secretary Douglas Alexander . He advocates taking a "hard headed view of Britain's interests", which means supporting plans to stabilise the single currency, while making sure single market rules are not skewed against the UK and pushing for reform to boost European trade. The balance of power between Westminster and Brussels is not ideal, Alexander concedes, but now is not the time to fixate on repatriation of powers.
That, as it happens is not so very far removed from the government's official negotiating position. The key difference is that Cameron actually has to deliver it and his raucous party has made it much harder for him (my column in this week's magazine deals with that in more depth). Meanwhile, Ed Balls has taken the lead for Labour in the economic debate around the single currency crisis and struck a slightly more sceptical tone. He has positioned the party firmly against British participation in any EU bailout funds. He also likes to take the credit for helping Gordon Brown keep Britain out of the single currency when Tony Blair wanted to join. Combined, the two positions make for a kind of cautious scepticism-lite - liking British membership of the EU for pragmatic commercial reasons; ready to like it more if the EU were something it is not.
That is a decent enough holding pattern. But it is not clear how it would evolve if Britain's EU relations lurch into a full-scale diplomatic crisis. Would Labour ever support Tory backbench calls for a referendum on a new EU treaty? The natural law of political opportunism dictates that they must denounce whatever deal Cameron does as a failure (which it probably will be), so should the party then join with Tory rebels and try to defeat it in parliament? If eurozone members proceed with their own fiscal consolidation, Britain's relationship with Brussels will, by definition, be changed. Will Labour then support calls for a more substantial renegotiation, including repatriation of powers? And, what it all comes down to in the end: how eurosceptic is Ed Miliband prepared to make Labour in order to make life really difficult for David Cameron and the coalition?
Labour's current position works as a cautious account of Britain's interests under the circumstances. But those circumstances are changing fast.