In this time of change our party must move beyond its comfort zone and find new ways of realising its age-old goals of equality and freedom.

The Labour Party’s greatest strength has always been our commitment to a society that is fairer and freer, more equal and more democratic. Our mission has always been to apply that commitment to the circumstances of our time. Today, when those circumstances are changing increasingly rapidly and old assumptions are breaking down, that task is tougher, and more pressing, than ever.

It is no longer simply enough to get into power and, from Whitehall, pull the old social-democratic levers: tax rates and regulation, welfare payments and tax credits. They have their place but these alone are inadequate to the task of delivering a fair, united society at a time when technology cycles are speeding up, new economic competitors are on the rise and the make-up and identity of our country are evolving. Living up to our age-old mission demands a willingness to grapple with the economic, social and global challenges as they are before us now.

Our movement faces three main, interlocking economic questions that are much more acute now than in the past: How to deliver excellent public services at a time when money is tight? How to harness the technological changes that are disrupting established industries and destroying jobs to create new, better opportunities? And how to remain competitive in a time of globalisation, paying our way in the world over the coming decades?

The answers, too, are interlocking. They include a more creative and strategic relationship between the state and business, a much better use of government’s convening power to channel investment into R&D and a genuinely life-long education system. The only sustainable foundation for Britain’s future prosperity is better productivity and a confident, adaptive workforce - not the private consumption and house price inflation on which our economy has too often been based in the past.

Our movement also faces newly urgent social questions. Building solidarity between different segments of the country is a bigger task today than it was in 1997. The very coherence of the United Kingdom is at risk. Regional identity is becoming more pronounced. In an increasingly diverse society, integration of different communities is ever-more essential. Our ageing population puts new pressures on public services and on the pact between the generations.

As the party that has always stood for a cohesive society, we in Labour must lead the charge in transforming the institutions of our country to keep up with evolving realities. That means reshaping the state: moving towards a more federal United Kingdom, devolving power and money to cities and regions, reforming our electoral system and political bodies to reflect the more open and pluralistic country they represent. And it means a new approach to public services: integrating health, mental health and social care services, starting at “what works” and putting the principles of prevention and innovation at the heart of the welfare state.

As the internationalists of British politics, those who see responsibilities and opportunities beyond our own borders, we in Labour must also champion a new approach to the world stage. Britain is no longer a recent superpower with an automatic claim to a place at the top table of nations, but a country with a moderately large population that must earn its right to that place. As CNN’s Fareed Zakaria wrote last week, Britain “has essentially resigned as a global power” under David Cameron - but is “even now, a country with the talent, history and capacity to shape the international order.”

So let our party lead this argument for a smart and engaged use of Britain’s vast soft power, its military expertise and specialisms and the versatility and reach that come from its unique network of alliances. We should, for example, fight to stay in the EU not just out of economic, transactional reasons but as part of a bigger, emotional, fundamentally optimistic vision of Britain’s future: as a country that chooses prosperity, security and geopolitical influence over an isolation that (as Zakaria concludes) would be bad not just for us, but for the world as a whole.

The job for the next Labour leader is to weave these imperatives, economic, social and global, into a credible national story of a country proud of its history and confident of owning the future. A vision of a Britain in which all can get on, whose citizens are financially secure and in control of their lives and happiness - and are, collectively, secure and effective in the wider world. A vision both rooted in the party’s eternal values and alive to the complexities and realities of the context in which we must now realise them.

For us, our next leader must get this vision right. On all these big subjects, Liz Kendall has asked the tough questions and started to chart a course to the answers. She has been courageous in challenging conventional wisdom. She has no compunction in moving Labour beyond our comfort zone and is determined to build a team ready to chart a route forward. This is exactly what our party needs and that is why we are nominating her to be the next leader of the Labour Party.

Chuka Umunna, Labour MP for Streatham & Shadow Business Secretary

Emma Reynolds, Labour MP for Wolverhampton North East & Shadow Communities Secretary

Jonathan Reynolds, Labour MP for Stalybridge and Hyde & Shadow Climate Change Minister

Stephen Twigg, Labour MP for Liverpool West Derby & Shadow Justice Minister