David Cameron's speech on the EU  was driven not by policy but by politics. Six months ago, the Prime Minister had no intention of promising an in/out referendum on the EU but his recalcitrant backbenchers and an insurgent UKIP forced him into a dramatic reverse ferret. His address, then, was less about outlining a sophisticated vision for the future of the EU (one that Cameron's fantasy of an à la carte Europe, in which Britain picks and chooses which rules it obeys, does not represent) but simply about getting him through the 2015 general election.
On that limited basis, the speech may prove to be a success. The early reaction from eurosceptic MPs, such as Douglas Carswell , suggests that it will help to unify a Conservative Party that has been badly divided over the EU since the election.
The biggest long-term problem for Cameron remains that having promised a "fundamental change" in Britain’s relationship with the EU, he will struggle to persuade the eurosceptics in his party that it is in our interests to remain a member if he fails to deliver. The result would be the worst Tory split for decades as some cabinet ministers, such as Iain Duncan Smith and Owen Paterson, argued for an 'out' vote, while others argued for an 'in' vote. But that, if Cameron wins a majority at the general election (and it remains a very large 'if'), is not an issue he will have to face until long after 2015.
For now, the Prime Minister enjoys the distinction of being the only party leader to have promised to give the electorate a vote over the EU at some point in the near future. This leaves Labour and the Liberal Democrats, both of whom have argued that Cameron's pledge is a rash one, in a difficult position. If they seek they match his offer at some point before 2015 (most likely in the form of a straight in/out vote, rather than one tied to renegotiation), they will look weak; following, not leading. If they do not, they will stand accused of denying the British people a say over an institution that has changed dramatically in the 38 years since the first and only EU referendum in 1975. Will Miliband and Clegg allow Cameron to be the only leader to stand up at the TV debates in 2015 and promise a referendum on the EU? Almost certainly not, which is why both must now work out how to climb down in the most graceful and painless way possible.