Dear Lib Dem voter

You might have voted for Nick Clegg on 6 May. But since the general election, I believe both the Liberal Democrat and Labour parties have been on a political journey. I think for both parties the destinations are now becoming visible.

Many who turned to the Lib Dems in 2010, or even those who turned to the party long before that, did so because they thought they saw a party that believed in fairness and social justice.

Indeed it is a party of proud traditions: of Keynes, Lloyd George and Beveridge. But it is increasingly clear that these traditions are being abandoned by Clegg as he goes along with damaging cuts in public spending undermining economic growth, tax rises hitting the poorest hardest, and a clear threat to the universal welfare state.

Our society is at risk of being reshaped in ways that will devastate the proud legacy of liberalism. We see a free market philosophy being applied to our schools, wasteful top-down reorganisation of our NHS, and the undermining of our green credentials with cuts to investment.

At some point you have to conclude that this is not a mistake here or there, but part of a pattern. The pattern is of a leadership that has sold out and betrayed your traditions, including that of your recent leadership: Steel, Ashdown, Kennedy and Campbell.

Since the election we have also seen the possible emergence of a changed Labour. We are proud of our record in government, from the children lifted out of poverty to the transformation of our NHS, but I believe I am winning the argument that we must turn the page on New Labour and the mistakes it led us to. For example, the argument is being won that a graduate tax based on income would be fairer than tuition fees and a market in higher education. The argument is being won that on issues like ID cards and stop-and-search we became too casual about the liberties of individuals. And I believe the argument is being conclusively won that we must recognise the profound mistake of the Iraq war.

I want to take my party on a journey to a different identity for the future: social democratic on economic policy, standing for redistribution and tackling inequality, liberal in our respect for individual rights.

That is a mission which contrasts with the mission Nick Clegg is taking you on: small state in respect of individual liberty, but small state too on economics. With me, you won't have to choose between whether to accept a reactionary assault on the welfare state in exchange for greater civil liberties. You can have both a commitment to equality and to liberty.

Thirty years ago next January, the Gang of Four issued the Limehouse Declaration that led ultimately to the SDP. This split the left of British politics for a generation. The formation and conduct of this coalition government gives an opportunity for a once-in-a-generation realignment of politics. I want to see Labour become home to a new progressive majority.

I believe that from this leadership contest you can see a party emerge which under my leadership shares your values. To the 1.5 million people who supported Labour in 1997 but have since then switched to support the Lib Dems, and to those who are long-term Lib Dem supporters, I ask you to look again at Labour.

If you join Labour you can come together with the emerging majority in the Labour party who want change that is real and lasting. If you join before 8 September you can vote to make that change in the leadership election.

Help build the society of equality, liberty and democracy that we all believe in, and stop the unfairness of this coalition. Leaving your party is the most honourable course when your party leadership leaves you.

Ed Miliband, the coordinator of the Labour manifesto and the former energy and climate change secretary and MP for Doncaster North.