For all the progress that new Labour has achieved on many fronts, the promise of a new political dawn, which inspired our dreams on 1 May 1997, stands in danger of being squandered. The very heart of the "project" - the promise not just of a new agenda, but of a new way of doing politics - has been forgotten. The central belief was that, in politics, means cannot be separated from ends. The end was a progressive 21st century, in stark contrast to the Conservative 20th century we were leaving behind. The means were a new approach to politics based on pluralism and co-operation. It was to be epitomised by closer collaboration between new Labour and the Liberal Democrats, rooted in a shared belief in equality, liberty and fraternity.
The project was about much more than electoral pragmatism. The opportunity was to heal the rift between progressive radicals that opened up a century ago. The danger now is that we shall be too tribal, too short-term and too timid to grasp the moment.
Social democrats and liberals stand, as Polly Toynbee has argued in the Guardian, on essentially the same ground: a presumption in favour of the underdog; a belief in progress (things can always get better); a trust in reason; taxing as much as you dare; tolerating almost anything except intolerance; striving for a more equal society; siding with the consumer; celebrating diversity; and regarding sex and family life as no business of the state. Above all, they both believe that all humans are redeemable.
If we removed our tribal blinkers, we would see that a mixture of modern social democracy and liberalism has rich political potential. A dose of liberalism applied to its centralist and statist mean streaks would do Labour a power of good. Equally, a dose of communitarianism and collectivism could temper the individualism that Lib Dems favour, but which easily slides into anomie. Each could act as both brake and spur to the other. After the end of communism, we are all liberals now. The question that counts is: are we conservative liberals, who put the needs of global capitalism first, or are we social liberals, who put the needs of people first?
So the ideological foundations of the project, we believe, are strong. The difficulty, as the past three years have shown, is to put them into practice, to achieve lasting change. From health to education, from welfare reform to the restructuring of local government, the gap between expectations and achievement is yawning. Take local government, where the transition from the Tories' competitive tendering to Labour's "best value" - a matter of detail, obscure to most outsiders - does little to address the loss of legitimacy, accountability and power that lies at the heart of the malaise in our town halls.
The fault resides not so much in particular government decisions as in our adversarial political system and the short-term pragmatism it inspires. Long-term change demands a long-term consensus. Progressive politics is demonstrably stronger when the strands of progressivism come together. Think of Labour's postwar construction of the welfare state, largely inspired by the Liberal William Beveridge, or its popular economic interventionism, largely inspired by the Liberal J M Keynes.
To achieve the prize of a progressive century, we have to give up the winner-takes-all approach of Westminster. Indeed, we have to recognise that the winner doesn't actually take all any more. Look at new Labour, sitting on a 180-seat majority but too often buffeted and constrained by forces beyond Westminster.
What unites us is vital; what divides us is trivial. Both our parties sometimes use leaflets and tactics that stink. Labour can be too pragmatic, the Liberal Democrats too principled. It is easy to decry control freaks when you have no control and easy to abuse power just because you have it. The point is how we can learn from each other. "Conservatives" are not just found in the Conservative Party. They lurk in Labour's undergrowth, eager to fix their way to power. They can be found too among the Liberal Democrats, vainly rejecting power and the compromises needed to achieve it.
Those in either party who want nothing to do with the other should recall that the tactical vote, apart from giving the Lib Dems a spectacular victory in the Romsey by-election this month, delivered about half the Liberal Democrat seats and one in seven Labour seats in 1997. If the tactical vote fails to materialise at the next election, Labour's overall majority would be instantly reduced to fewer than 60, even if the Tories failed to get a single extra vote. And in using the tactical vote, the voters have shown that they are ahead of the politicians - they prefer a more adult relationship in which political parties work together where they can agree.
Labour and the Liberal Democrats showed themselves capable of this during the long years of opposition. For example, they made a deal on constitutional reform that has been steadily implemented in this parliament. Though the Liberal Democrats might like the reforms to go further, and some in Labour's ranks might think constitutional change has taken up too much time, the important point is that the changes have been popular - so popular that it is now inconceivable for the Tories to reverse devolution, abolish the Greater London Authority or bring back hereditary peers.
The two parties and their leaders could certainly move forward together on this front. Before an election, we should be able to agree on a reformed and democratic House of Lords, a progressive Freedom of Information Act and the need for a referendum in the next parliament on electoral reform. This would open up the prospect of agreeing further change to strengthen local and regional government. The two parties are also close in their principles on Europe, race relations, international development and rural affairs - all these areas would be worthy of discussion.
In the longer term, we could build consensus around a responsible return to tax-and-spend policies. This would be the basis for spreading wealth and opportunity more evenly not just through straightforward redistribution, but also through reformed public services in which people would have renewed confidence. We could also build on the employment rights established since 1997 - something that trade unionists should bear in mind as they consider their position on electoral reform.
We do not support a formal pact or merger. Lib-Lab co-operation does not need one big tent, merely a common campsite. New politics means knowing when to co-operate and when to compete. The Liberal Democrat leader Charles Kennedy has made it clear that Labour is the competition, but the Tories are the enemy. On some issues such as social justice, the two parties should compete more aggressively in who can offer most and deliver most. Even John Prescott must have been secretly pleased that one of the big vote-winners for the Lib Dems this month in places such as Romsey, Oldham and Liverpool was that Labour had increased pensions by only 75p. And would any Liberal Democrat MP not applaud Gordon Brown's announcement of increased spending on the NHS?
We are seeking a fundamental change in the culture of our politics - and that is the very hardest thing to change. But time is pressing. Centre-left politicians need to rediscover a sense of vision and ambition and to convey to the electorate that tactical voting is not just a device to get the Tories out as it was in 1997; it can also be the means of delivering progressive ideals. Our two parties should implicitly agree that they will not waste scarce election resources in seats that only one of us can win. Officially, local pacts and tactical voting will not be condoned, but it should be possible to turn a blind eye. Wherever possible, party activists should target the Tories, not each other. And both leaders must make clear that tactical voting on election day will be reflected in co-operative and pluralist politics the day after.
We are not alone, it would seem, in our regret over the missed opportunities to forge a progressive alliance. In a recent interview, Tony Blair looked back at May 1997 and the missed chance for a formal realignment of the centre-left. The time to be brave and to follow his pluralist instincts has come again. He is unlikely to get another chance. As the polls show a narrowing gap against the Tories, as the lessons of Romsey sink in and as Kennedy grows into his job, we detect a growing recognition near the top of our two parties that the project should come out of cold storage. But if it is to work in the long term, it must prioritise fundamentals over fixing.
Blair is not a tribal politician; and Kennedy has a canny eye on power. Between them, they hold the cards of a winning progressive hand. The search for a new form of social capitalism is bigger than either party - and probably out of reach if each of them obstinately decides to go it alone. The early months and years of the 21st century will determine whether or not it can be a progressive one.
Neal Lawson, a former adviser to Gordon Brown, is managing editor of the new Labour quarterly, Renewal. Neil Sherlock fought the 1992 and 1997 elections for the Liberal Democrats. With the support of the Joseph Rowntree Reform Trust, they are editing a book on prospects for a progressive century