There is only one iron-clad law of political history - the law of unintended consequences. When, in 2009, David Cameron urged Gordon Brown to sign up to a televised leaders' debate, he no doubt envisaged that such a broadcast would serve to draw voters' attention to the contrast between himself, the "heir to Blair", and an exhausted Prime Minister. As it turns out, he has succeeded merely in handing a historic opportunity to the Liberal Democrats.

Nick Clegg is important not for what he says but for what he represents. As the former Sun editor David Yelland has pointed out, suddenly, "the powerless have a voice" (one that had previously been silenced by the might of the two main party machines, in collusion with Rupert Murdoch's media empire). We welcome this development. For some time now, we have been arguing in our leaders that a hung parliament offers the prospect of a realignment of progressive politics in this country on the basis of a common programme of fundamental constitutional and economic reform, stronger civil liberties and an enhanced Freedom of Information Act.

A month ago, before the campaign began, we wrote that it "may fall to Mr Clegg to resolve the historic 'progressive dilemma' in British politics". This is no longer merely wishful thinking - but only if Labour and the Lib Dems are ­prepared to form a partnership of principle, as they should have done after the 1997 landslide election victory, a partnership founded on a desire to clean up and repair our broken political system.

On page 22, in their contribution to our ongoing series of essays on the future of progressive politics, Richard Reeves and Dan Leighton argue that it is the Lib Dems' attacks on "vested interests in Westminster and the City" that have caught the imagination of voters. They are right: this is not anti-politics but possibly the beginning of a new politics - a vision of the wholesale transformation of our decrepit and dysfunctional late-Victorian and Edwardian institutions. Electoral reform, the introduction of a written constitution, a recall system for MPs found guilty of serious wrongdoing - these are all policies in the Liberal Democrat manifesto that speak to what can be called a "republican moment".

Mr Cameron has promised to introduce a "big society" that would empower individual citizens, though his enthusiasm for devolving power didn't extend to the City of London, the other centre of untramelled and unaccountable power in this country. The Lib Dems, by contrast, like Labour understand the need for firmer regulation of the financial sector. (In this, they can, as Amartya Sen reminds us on page 29, call upon no less an authority than the great Adam Smith.) Their manifesto includes plans for a much-needed separation of retail banking from investment banking.

The Lib Dems are bold on taxation, too. We applaud their plans to tax capital gains at the same rate as personal income; for too long, the rich have abused the capital gains tax system. Their promise to raise the tax threshold to £10,000 would mean 3.6 million people on low incomes would pay no income tax. (The picture is spoiled by the Lib Dems' commitment to abolishing the Child Trust Fund; even the Tories propose keeping the fund for the poorest third of families.)

There is much here that will appeal to progressive, centre-left voters. Mr Clegg, however, continues to maintain the fiction of "equidistance" from the other two main parties. This is understandable, for tactical reasons. But it is nevertheless clear that the Labour Party remains the more natural ally of the Lib Dems - a point that the Transport Secretary, Andrew Adonis, made early in the campaign, well before the onset of "Cleggmania". "Equidistance", he said, is a "nonsense". We agree. After all, it was Beveridge and Keynes, Liberals both, who wrote the blueprint for the welfare state.

It is important to note that Lord Adonis's remarks were directed at his own party as well as at the Lib Dems. They were a challenge to his colleagues to put reactionary, tribalist "Labourism" behind them and to embrace pluralism of a kind advocated by the free-thinking Labour MP Jon Cruddas.

The Prime Minister is now calling for a progressive "anti-Tory alliance" - just as the clock is about to strike midnight. Mr Brown, like his struggling party, is a belated convert to the merits of such a coalition; there is some truth in Mr Clegg's claim that the Prime Minister has previously stood in the way of constitutional reform.

Ultimately, however, suspicions about Mr Brown's motives are beside the point. This is too important an oppor­tunity to squander.