British politics could be at a major turning point. Take a look at some basic facts: in the 1951 general election, only 2 per cent of voters supported parties other than the Conservatives and Labour. At the local elections last year, that figure had shot up to 40 per cent. During the last two general elections, for the first time in post-war Britain, more people didn't vote at all than voted for the winning party. Gordon Brown is Prime Minister even though only 22 per cent of eligible voters supported the Labour Party at the last election; at the same election, almost one in four of those people who voted – close to six million voters – supported the Liberal Democrats, yet only 10 per cent of MPs are Lib Dems.

I think these facts tell an important story: the old duopoly of British politics is dying on its feet, but our institutions still operate as if nothing has changed since the height of the Cold War. An increasing number of people are voting with their feet by voting for other parties, or not at all.

Politics is stuck in a no-man's-land, caught between the reality of greater pluralism and the conservativism of duopoly politics. Millions of people no longer feel any allegiance to the red-blue blue-red politics of old, yet our electoral system and Parliament sustain the fiction that the two old parties still speak for Britain.

Not surprisingly, the Labour and Conservative parties seek to ignore all this. I see it every week at Prime Minister's Questions – the braying, the hyperbolic claims and counter- claims, the lame jokes, each party gaining a few inches of advantage in a game of political attrition which leaves the rest of the country cold. Like vested interests in the commercial world, the old parties stitch up the rules to keep others out.

That is the unspoken story behind the expenses scandals. There are hundreds of Conservative and Labour MPs who have been given jobs for life in safe seats even though only a minority of their constituents support them. A third of seats in the House of Commons haven't changed hands since the Second World War. Power without accountability, privilege without competition always leads to trouble. No wonder they abused the expenses system on an industrial scale – flipping from one taxpayer-funded home to the next for personal gain, avoiding paying capital gains tax.

This election could be an election of renewal, when the old politics finally passes its sell-by date and a new era of pluralism and accountability is ushered in. The one advantage of a crisis – economic, political, social – is that it can open the door to a new way of doing things. It can make the unthinkable thinkable, the idealistic realistic. It can be the beginning of something new. Yet what are the old battalions of the Conservative and Labour parties doing? Turning their fire on each other in a frenzy of mutual loathing.

The Conservative Party strategy is now clear: personal animus towards its opponents; shameless scaremongering in the financial markets; double standards in its own policies. David Cameron's spring conference speech carried one message only: vote for me, because I really really hate the other guy. George Osborne's economically illiterate warnings of meltdown in the money markets carried one message only: vote for us otherwise we'll get the markets to tear the house down. And their supine defence of Lord Ashcroft's tax status carried one message only: there's one rule for the little people, and another rule for us. If David Cameron really wanted to prove that he believes in a new politics, he should have sacked Lord Ashcroft as vice chairman of the Conservative Party long ago.

Meanwhile, the Labour Party can't believe its luck: with one bound Gordon Brown and Peter Mandelson are free from defending the indefensible – 13 years of broken promises, a record of betrayal of what progressive politics is supposed to be about. Unregulated greed in the banks. Inequality up, social mobility down. The mass incarceration of young people. The decimation of our civil liberties. A political system in crisis. No wonder they prefer the trench warfare of point-scoring instead. Gordon Brown lumberingly sets out his dividing lines: implausible pledges which no one believes to keep on spending and spending; a promise of free personal care for the elderly which will in truth deprive many vulnerable people of the care they desperately need; a deathbed conversion to the most minimal changes possible to our electoral system; a one-off levy on bankers' bonuses while the banks cut lending to viable British businesses; a laughable slogan to deliver "a future fair for all" as unfairness deepens everywhere.

Gone from both parties is any sense of a coherent ideology, or a plan for governing Britain. They have their sights focused on 6 May, not on the a wider vision of what kind of society we want to be.

But it really doesn't need to be like this. Just because the old parties choose to cling to the politics of spite doesn't mean the rest of us can't hope and strive for something better. Liberal Democrats remain determined to transform British politics, once and for all. We want to create a plural, vibrant politics where everybody's voice is heard and every vote counts, where politics is a battle of ideas, not a contest of advertising budgets. The old parties are locked in a battle for knock-out supremacy, when people in modern Britain are crying out for something new.

Gordon Brown and David Cameron will tell you that the only choice is between left and right, between red and blue. But people no longer believe there are only two answers to every problem. Modern Britain is diverse, mobile and changing fast. The cramped duopoly that has dominated our politics for so long no longer reflects the varied aspirations, hopes and ambitions of modern Britain.

In truth, there is a bigger choice to make today: between the old politics and the new.