Citizens have votes. Newspapers do not. However, if the Guardian had a vote in the 2010 general election it would be cast enthusiastically for the Liberal Democrats. It would be cast in the knowledge that not all the consequences are predictable, and that some in particular should be avoided. The vote would be cast with some important reservations and frustrations. Yet it would be cast for one great reason of principle above all.

After the campaign that the Liberal Democrats have waged over this past month, for which considerable personal credit goes to Nick Clegg, the election presents the British people with a huge opportunity: the reform of the electoral system itself. Though Labour has enjoyed a deathbed conversion to aspects of the cause of reform, it is the Liberal Democrats who have most consistently argued that cause in the round and who, after the exhaustion of the old politics, reflect and lead an overwhelming national mood for real change.

Proportional representation – while not a panacea – would at last give this country what it has lacked for so long: a parliament that is a true mirror of this pluralist nation, not an increasingly unrepresentative two-party distortion of it. The Guardian has supported proportional representation for more than a century. In all that time there has never been a better opportunity than now to put this subject firmly among the nation's priorities. Only the Liberal Democrats grasp this fully, and only they can be trusted to keep up the pressure to deliver, though others in all parties, large and small, do and should support the cause. That has been true in past elections too, of course. But this time is different. The conjuncture in 2010 of a Labour party that has lost so much public confidence and a Conservative party that has not yet won it has enabled Mr Clegg to take his party close to the threshold of real influence for the first time in nearly 90 years.

This time – with the important caveat set out below – the more people who vote Liberal Democrat on 6 May, the greater the chance that this will be Britain's last general election under a first-past-the-post electoral system which is wholly unsuited to the political needs of a grown-up 21st-century democracy.

Tactical option

The pragmatic caveat concerns the danger that, under the existing electoral system, switching to the Liberal Democrats in Labour-Conservative marginal constituencies might let in an anti-reform Tory party. So, voters who share this principled enthusiasm for securing the largest possible number of Liberal Democrat MPs next Thursday must, in many constituencies, weigh the tactical option of supporting Labour to prevent a Conservative win.

Hopefully, if this really is the last election under the old system, such dilemmas between head and heart will apply less in future. For now, however, the cause of reform is overwhelmingly more likely to be achieved by a Lib Dem partnership of principle with Labour than by a Lib Dem marriage of convenience with a Tory party which is explicitly hostile to the cause and which currently plans to redraw the political map for its own advantage. The momentum for change would be fatally undermined should the Conservatives win an overall majority. The Liberal Democrats and Labour should, of course, have explored much earlier and more explicitly how they might co-operate to reform the electoral system. During the campaign, and especially since the final leaders' debate, the appetite for co-operation has clearly increased and is increasing still. Mr Clegg's Guardian interview today underscores the potential for more productive engagement with Labour and is matched by fresh, untribal thinking from his potential partners.

This election is about serious choices between three main parties which all have something to offer. David Cameron has done what none of his immediate predecessors has understood or tried to do: he has confronted the Conservative party with the fact that it was out of step with the country. He has forced the party to become more diverse and to engage with centre-ground opinion. He has explicitly aligned himself with the liberal Conservative tradition which the Thatcherites so despised during their long domination of the party. He has promoted modern thinking on civil liberty, the environment and aspects of social policy.

Mr Cameron offers a new and welcome Toryism, quite different from what Michael Howard offered five years ago. His difficulty is not that he is the "same old Tory". He isn't. The problem is that his revolution has not translated adequately into detailed policies, and remains highly contradictory. He embraces liberal Britain yet protests that Britain is broken because of liberal values. He is eloquent about the overmighty state but proposes to rip up the Human Rights Act which is the surest weapon against it. He talks about a Britain that will play a constructive role in Europe while aligning the Tories in the European parliament with some of the continent's wackier xenophobes. Behind the party leader's own engagement with green issues there stands a significant section of his party that still regards global warming as a liberal conspiracy.

The Tories have zigzagged through the financial crisis to an alarming degree, austerity here, spending pledges there. At times they have argued, against all reason, that Britain's economic malaise is down to overblown government, as opposed to the ravages of the market. Though the Conservatives are not uniquely evasive on the deficit, a large inheritance-tax cut for the very wealthy is the reverse of a serious "united and equal" approach to taxation. Small wonder that the Cameronisation of the Conservative party sometimes seems more palace coup than cultural revolution. A Cameron government might not be as destructive to Britain as the worst Tory regimes of the past. But it is not the right course for Britain.

If this election were a straight fight between Labour and the Conservatives – which it absolutely is not – the country would be safer in the hands of Labour than of the Tories. Faced in 2008 with a financial crisis unprecedented in modern times, whose destructive potential can hardly be exaggerated, the Labour government made some absolutely vital calls at a time which exposed the Conservatives as neoliberals, not novices. Whether Labour has truly learned the right lessons itself is doubtful. Labour is, after all, the party that nurtured the deregulatory systems which contributed to the implosion of the financial sector, on which the entire economy was too reliant. How, and even whether, British capitalism can be directed towards a better balance between industry and finance is a question which remains work in progress for Labour, as for us all. At the highest levels of the party, timidity and audacity remain in conflict. Nevertheless, Labour, and notably Alistair Darling, a palpably honest chancellor who has had to play the most difficult hand of any holder of his office in modern times, deserves respect for proving equal to the hour. Only the most churlish would deny the prime minister some credit for his role in the handling of the crisis.

Labour's failings

But this election is more than a verdict on the response to a single trauma, immense though it was. It is also a verdict on the lengthening years of Labour government and the three years of Gordon Brown's premiership. More than that, any election is also a judgment about the future as well as a verdict on the past. A year ago, the Guardian argued that Labour should persuade its leader to step down. Shortly afterwards, in spite of polling an abject 15.7% in the European elections, and with four cabinet ministers departing, Labour chose to hug Mr Brown close. It was the wrong decision then, and it is clear, not least after his humiliation in Rochdale this week, that it is the wrong decision now. The Guardian said a year ago that Mr Brown had failed to articulate a vision, a plan, or an argument for the future. We said that he had become incapable of leading the necessary revolution against the political system that the expenses scandal had triggered. Labour thought differently. It failed to act. It thereby lost the opportunity to renew itself, and is now facing the consequences.

Invited to embrace five more years of a Labour government, and of Gordon Brown as prime minister, it is hard to feel enthusiasm. Labour's kneejerk critics can sometimes sound like the People's Front of Judea asking what the Romans have ever done for us. The salvation of the health service, major renovation of schools, the minimum wage, civil partnerships and the extension of protection for minority groups are heroic, not small achievements.

Yet, even among those who wish Labour well, the reservations constantly press in. Massive, necessary and in some cases transformational investment in public services insufficiently matched by calm and principled reform, sometimes needlessly entangled with the private sector. Recognition of gathering generational storms on pensions, public debt, housing and – until very recently – climate change not addressed by clear strategies and openness with the public about the consequences. The inadequately planned pursuit of two wars. A supposedly strong and morally focused foreign policy which remains trapped in the great-power, nuclear-weapon mentality, blindly uncritical of the United States, mealy-mouthed about Europe and tarnished by the shame of Iraq – still not apologised for. Allegations of British embroilment in torture answered with little more than a world-weary sigh. Large talk about constitutional change matched by an addiction to centralisation. Easy talk about liberty and "British values" while Britain repeatedly ratchets up the criminal justice system, repeatedly encroaches on civil liberties, undermines legal aid and spends like there is no tomorrow on police and prisons. Apparent outrage against the old politics subverted by delay, caution and timid compromise.

There are reservations too, though of a different order and on different subjects, about the Liberal Democrats. The Liberal Democrats are a very large party now, with support across the spectrum. But they remain in some respects a party of the middle and lower middle classes. Labour's record on poverty remains unmatched, and its link to the poor remains umbilical. Vince Cable, so admirable and exemplary on the banks, nevertheless remains a deficit hawk, committed to tax cuts which could imply an even deeper slashing of public services. Though the party has good policies on equality, it has not prioritised the promotion and selection of women and ethnic minority candidates.

Matched priorities

Surveying the wider agenda and the experience of the past decade, however, there is little doubt that in many areas of policy and tone, the Liberal Democrats have for some time most closely matched our own priorities and instincts. On political and constitutional change, they articulate and represent the change which is now so widely wanted. On civil liberty and criminal justice, they have remained true to liberal values and human rights in ways that the other parties, Labour more than the Tories in some respects, have not. They are less tied to reactionary and sectional class interests than either of the other parties.

The Liberal Democrats were green before the other parties and remain so. Their commitment to education is bred in the bone. So is their comfort with a European project which, for all its flaws, remains central to this country's destiny. They are willing to contemplate a British defence policy without Trident renewal. They were right about Iraq, the biggest foreign policy judgment call of the past half-century, when Labour and the Tories were both catastrophically and stupidly wrong. They have resisted the rush to the overmighty centralised state when others have not. At key moments, when tough issues of press freedom have been at stake, they have been the first to rally in support. Above all, they believe in and stand for full, not semi-skimmed, electoral reform. And they have had a revelatory campaign. Trapped in the arid, name-calling two-party politics of the House of Commons, Nick Clegg has seldom had the chance to shine. Released into the daylight of equal debate, he has given the other two parties the fright of their lives.

A newspaper that is proudly rooted in the liberal as well as the labour tradition – and whose advocacy of constitutional reform stretches back to the debates of 1831-32 – cannot ignore such a record. If not now, when? The answer is clear and proud. Now.

• More Guardian election comment from Cif at the polls