It had been just another hung-parliament day of clandestine comings and goings, more carefully worded communiqués, and further rounds of quibbling about small print. Then Gordon Brown emerged from No 10 and, with a brisk five-minute statement, changed the whole nature of the game.
In announcing his intention to resign as Prime Minister and leader of the Labour Party, Mr Brown bowed to the inevitable. He had led his party to defeat in the election; until a late rally in the final days, he had fought a lacklustre campaign. That he was still in Downing Street at all reflected the constitutional perversity of a result that had produced no overall majority. Until someone can cobble together a majority – or the Conservatives are left to try their luck as a minority government – Mr Brown has little choice but to soldier on.
The accusation that he was a "squatter" in his own home was partisan mischief-making of the first order – but mischief-making that clearly hurt. In spelling out that he would depart, once there was a new government and a new party leader, he may have sought to dispel the image of a lonely autocrat stubbornly clinging on to power. Without doubt, though, he also sought to pre-empt those on his own side calling for him to leave.
In one way, this represented a return to the natural order of things. Here was the concession that a defeated party leader is expected to make in the early hours of the morning after the election. Without a clear result, this was not possible. Even so the acceptance of defeat could be said to have come four days late.
Yet no one can reproach Mr Brown with casually throwing away the little political capital that remained to him. The timing of his declared political hara-kiri was exquisite; it instantaneously upset calculations about a Conservative-Liberal Democrat alliance that were starting to be taken for granted. Gordon Brown can truly be said to have sacrificed himself for his party.
With Mr Brown's days in office now officially numbered – though perhaps not as few as his detractors had hoped – everything is suddenly back in play. And that includes an arrangement between Labour and the Liberal Democrats and the few others needed to form what would essentially be an anti-Conservative government – or, as Mr Brown called it, a "progressive" alliance.
That many Liberal Democrats are more comfortable with the idea of joining Labour, rather than the Conservatives, has always been clear. The term "progressive" has its appeal. But Nick Clegg was taking a pragmatic, as well as principled, approach in allowing the Conservatives to get their courtship in first. The arithmetic was unambiguous: only the Conservatives could form a majority with the Liberal Democrats. If either the Liberal Democrats or Labour had done just a little better than they did last Thursday, Mr Clegg would have had more options in his aspirations to be kingmaker.
Yet the overriding question for the Liberal Democrats was – quite rightly – how much real ground Mr Cameron would give on their central issue: electoral reform. It was all very well for the Conservative leader and his lieutenants to keep citing the "national interest", but the election, and even more its fractured result, conclusively made the case for reform being in the national interest, too.
That his party held the balance of power gave Mr Clegg an opportunity that arises only once, if that, in a generation. Thanks largely now to Gordon Brown, that opportunity is still there. Last night, the Conservatives produced their "final offer" – a referendum on AV, the least ambitious electoral reform introducing the alternative vote. Mr Clegg is entitled to see whether Labour is prepared to offer something better.
Lord Ashdown described Mr Clegg's dilemma as "a deliciously painful torture mechanism", and he was not far wrong. But it has been eased, just a little, by Mr Brown's resignation. He now has more to gain and less to lose, whatever choice he makes.
As Prime Minister, Mr Brown made many mistakes. He was elevated without a leadership contest; he ducked holding an election to legitimise his position, and he did not cede power when it could have improved Labour's election chances. But the manner of his departure suddenly opens up British politics. It is a timely and fitting bequest.