Only last Wednesday, the Prime Minister was squirming on the front bench as Ed Miliband taunted him for being weak on Europe. Today, David Cameron will be cheered by his own side as the Labour leader taunts him for being strong on Europe. The only man squirming on the front bench will be the Deputy Prime Minister, Nick Clegg. For Clegg has had to execute the most uncomfortable volte face. On Friday morning, journalists were being briefed that he had been in constant touch with Cameron throughout the night and that he supported the veto. Now it transpires that he was fast asleep in his Sheffield flat, discovered about the veto only as a fait accompli, and is "bitterly disappointed" by it.
The mood among the Liberal Democrats is as torrid as it was after they lost the referendum on electoral reform. "There's a lot of bad temper flying around," says one Lib Dem Cabinet minister. "It was a big mistake." Clegg saw the horror in his own party and realised that he had to say what he really thought.
From now on, it looks as if on Europe, members of the Coalition will be allowed to disagree. As one senior Lib Dem puts it: "One of the great dividing lines in British politics now is between those who think the summit was a success and those who think it was a complete failure. From our point of view it was a shocking failure: a bad day for the Government, a bad day for Britain and a bad day for Europe." That divide will be seen in the Commons today: Tories jumping up to congratulate the Prime Minister and Lib Dems expressing grave reservations.
Clegg's outburst on The Andrew Marr Show yesterday was also designed to give breathing room to his Lib Dem Cabinet colleagues. There is no question of Vince Cable resigning – a rumour that flitted around the Twittersphere yesterday – as the Business Secretary's views have now been publicly shared by his leader. Nor is there any question of the Coalition breaking up. The Lib Dems are adamant that it was formed to deal with an economic crisis, and the crisis is by no means over.
But how will two parties so fundamentally at odds be able to take decisions when new questions about the EU arise? So far the technique has been, as far as possible, to avoid even talking about European policy. At Cabinet, it has barely been discussed. Usually, Cabinet meetings are cordial affairs in which there is cross-party agreement on deficit-cutting and welfare reform. The only time they touch on Europe is when George Osborne gives a summary of the euro situation, after which Clegg always blames the Germans and Ken Clarke says it's a big mess. And that's that.
Tomorrow's meeting will surely be different. Fresh from a Commons triumph, Cameron will be backed by all but one of his party colleagues. It will be Clegg, Cable, Chris Huhne, Danny Alexander and Ken Clarke against the rest. And the Lib Dems themselves are divided on the nuances. Clegg agrees with Cameron that the City must be protected against more EU regulation. Cable thinks that Cameron is focusing too much on financial services when the Government is supposed to be rebalancing the economy.
Bizarrely, the best hope for resolving their Coalition differences is a collapse of the eurozone, though no minister dare say it out loud. They all suspect that even the fiscal compact agreed last week is unlikely to solve the eurozone's immediate problems. One politician compares it to two people coming across a man bleeding in the gutter and earnestly discussing where to build a hospital. Markets will decide whether the eurozone survives long before any new fiscal rules have time to clear parliaments and pass referendums.
And if the euro doesn't endure in its present form, euro-enthusiasts will have to rethink fundamentally their view of Britain's relationship with Europe. Instead of their reflex assumption that Britain suffers from being "isolated", they might start to appreciate the security, flexibility and independence that we have gained from staying out of this dangerously ill-conceived project. If a herd is hurtling towards a cliff, after all, the one who peels away saves his life.
Even the Lib Dems recognise this. They don't want the euro to collapse, but if it does, at least party members will no longer be clamouring for us to be at the heart of Europe. And the British exercise of the veto, which so embarrassed them, will be completely overshadowed.
What of the Tory eurosceptics? They are, of course, delighted by Cameron's move. One minister woke up on Friday to hear of it on the Today programme: "I was flabbergasted! I wandered round the whole day thinking 'this is a completely different world,' after so many years of British prime ministers selling us down the river on Europe."
But Cameron can't afford to indulge them too much. For a start, it will put immense pressure on his Coalition partners if his tone is too triumphalist. More important, the events of Thursday night and Friday morning showed just how hard any further negotiation would be. If Cameron could not persuade his European partners to indulge him on what he saw as a moderate protection for the City, how on earth will he convince them to repatriate powers to the UK? Unless, that is, the whole architecture of the EU is being redesigned after a collapse of the euro. Then the logic of ever-closer union would have been tested to destruction. Britain might well find allies, in those circumstances, for an EU based on looser ties that is far more congenial to his party and voters.
The leader with the hardest task is Ed Miliband. He knows that the British public is naturally Eurosceptic: 62 per cent, in a Mail on Sunday poll yesterday, approved of Cameron's veto. Voters may not buy his line that Britain will suffer from not being "in the room" when they see how disastrously those in the room are faring. Nor will he look very credible when he fails to answer the question of whether he would have signed the treaty on Friday. "I wouldn't have started from there" is both true and accurate, but it is hard to sell politically. All Miliband can do is criticise Cameron for having taken the Tories out of the European People's Party grouping that would have won them allies, and for having failed to do enough groundwork before the negotiations.
As ever in politics, external events will have far more influence on all parties' fortunes than any meticulously designed policy. Only the most sceptical of Tories are actively willing a collapse of the euro,because of the economic pain it would cause. But it could have the side-effect of making government easier and the UK's position in Europe a lot more comfortable.