Andrew Adonis: It's madness to split the centre-left vote

Nick Clegg will spend the next month attempting to cast a "plague on both your houses". The truth is that the Lib Dems, for all their local opportunism, have national policy that is similar to Labour's. The difference is that Labour can implement its programme. The Lib Dems have no realistic chance to implement theirs without a Labour government. In Labour-Tory marginals, a vote for the Lib Dems is a vote which helps the Tories against progressive policies. And in Labour-Lib Dem marginals every Labour MP returned is a seat in the Commons more likely to put Labour ahead of the Tories and therefore better placed to form a government.

Philosophically it is a nonsense to pretend that the Lib Dems – or the "Social and Liberal Democrats" to give the party its original name – are equidistant between left and right, or Labour and Tory. The Liberal party of Gladstone, Asquith and Lloyd George fought the Tories relentlessly to introduce democracy and social rights. Keynes and Beveridge – Liberals both – produced the rationale and the blueprint for the modern welfare state enacted by Attlee's Labour government after 1945.

The Liberals won their last election precisely a century ago. Or rather they were the governing party in a hung parliament which, with strong Labour support, introduced national insurance for the sick and unemployed, abolished the veto of the hereditary House of Lords, raised taxes on the rich, and permitted the trade unions to support political parties financially. In turn the Liberals supported the 1924, 1929 and 1977 minority Labour governments. Until recently the two parties were in coalition in Scotland. Outside wartime, the official Liberal party has never supported a Tory government. Lloyd George's decision to govern with the Tories after 1918 – even with the plea of a post-war national emergency – split his party, destroyed his own political authority, and led directly to Labour replacing the Liberals as the major party of the left within five years.

A similar fate would inevitably befall Nick Clegg were he ever to copy Lloyd George, as the briefest visit to a Lib Dem conference testifies. Roy Jenkins told me when I was considering joining Labour after Tony Blair became Labour leader: "The only real difference is that Labour is now the larger party of social democrats, the Lib Dems are the smaller; and in our political system, it is generally wise to support the larger party if they are on the same page."

Unsurprisingly, given their similar values, the two parties share largely similar policies. Apart from the issue of proportional representation for elections to the Commons, where the Lib Dems have an obvious vested interest, and Iraq (a bitter disagreement but now largely behind us), the Lib Dems have not set out fundamental differences of principle with New Labour.

The Lib Dems have supported our investment in the public services, radical constitutional reform, equal rights, fair taxation, environmental protection and positive engagement in Europe. As his diaries reveal, Paddy Ashdown ardently sought a coalition with Labour after 1997, which might have happened had the Labour majority not been so large.

Take political reform, which is top of the Liberal agenda. It was this Labour Government that devolved power to a Scottish Parliament, a Welsh Assembly, a Northern Ireland Assembly and a directly elected Mayor of London. It was this Labour Government that introduced the Human Rights Act and that finally passed a Freedom of Information Act, giving the public the right to know. It was this Labour Government that removed most of the hereditary peers from the House of Lords – a reform the Liberals failed to implement when they had the chance in 1911. And it was the Tories who this week blocked a referendum on voting reform and our legislation to remove the remaining hereditary peers.

All these policies were supported by the Lib Dems. Yes, in some cases they were Lib Dem policy before Labour embraced them. But it took a Labour Government to implement change, working in tacit partnership with the Lib Dems. In his foreword to the 2005 Lib Dem manifesto, Charles Kennedy wrote: 'Society is still scarred by inequality. Tackling that is a priority for the Liberal Democrats... We remain proud of a Britain which is enriched precisely because it is a multi-racial, multi-ethnic society. We will not pander to fear and prejudice... our instinctive internationalism is definitive." Gordon Brown would say amen to all that.

Most likely it will be the same with the manifestos for this election. Over the past week we have stood together against the reckless Tory plan to cut £6bn from public services and reverse the NI increase. We both oppose David Cameron's inheritance tax cuts for the rich. We both pledge credibly to invest in the high-tech economy and protect and improve education, health, support for children and pensioners, and public transport. Neither of us will disengage from Europe. We both want to see a fully elected second chamber and a referendum of voting reform for the Commons.

Over recent months I have engaged closely with Norman Baker, the Lib Dem transport spokesman, on plans for high speed rail between London and Scotland – a transformational policy for future prosperity and national cohesion. The Tories refused to engage, even though they claim to support high-speed rail.

Of course there are differences. On criminal justice and asylum, the Lib Dems give a different weight to rights and responsibilities, and they favour more proportional voting systems, although with Labour's support for a referendum on the alternative vote this is now a debate about the nature rather than the principle of reform. Labour continues to be a broader social coalition, including the trade unions and business. Yet these differences are dwarfed by our common antipathy to a Tory party which, for all its glossy branding, remains wedded to the 1980s in economic, social and European policy.

To avoid a Tory government after May 6th, it is vital to grasp now the fundamental Labour-Lib Dem identity of interest. This can best be served by Labour coming out of the election as strong as possible, able to form a government.

Lord Adonis is Transport Secretary and Editor of 'Roy Jenkins: A Retrospective'