Labour’s leadership contest starts on Tuesday. But the scale of December’s election defeat is a challenge to all of us in the party, not just our next leader. So here are seven things the whole Labour party must now do.

1 Face the scale of defeat with humility

We didn’t win the argument, we lost. For the fourth time in a row. The Tory party has more power now than for a quarter of a century – power to hurt the families, communities and public services we were fighting for.

Our socialist party lost the majority of the working-class vote and fell behind in the very industrial towns that forged Labour and were hardest hit by Tory austerity. Labour values on equality, solidarity and social justice matter more than ever, and many people are crying out for a progressive government. But we couldn’t persuade people this time and unless we get our act together, we could lose even more support in future.

It wasn’t just about perceptions of Jeremy Corbyn. Or Brexit. Or scepticism about our manifesto. It was about the fact that on every one of those issues a gulf has grown between the party and the voters we lost. The next leader has to bridge that gap but they won’t stand a chance of doing so if the rest of us pretend it doesn’t exist.

2 Stop the factional infighting

We cannot be a narrow hard-left party. That doesn’t reflect our values or history. Nor will we win next time if we collapse into polarised factional infighting. Parties are teams. If we can’t compromise with each other, we can’t hold any coalition of voters together.

I stood against Jeremy last time – and while I agreed with him on tackling inequality and climate change, I criticised him on security and credibility. I was proud to be a minister in the last Labour government – a government that some current shadow cabinet members have strongly criticised.

As a result, whether fair or not, I recognise there are many in our party who won’t see me as the person to pull all sides of the party together following Jeremy’s departure. And rebuilding towards a Labour government is more important than any one person. So I am really grateful to everyone who urged me to stand again but this isn’t the time for me to do so. However, I’m not going anywhere – the next leader needs support and we all need to keep fighting for the Labour government our country so badly needs.

3 Be a party for the whole country – not just a liberal-labour party for the cities

Being a broad church isn’t just about left and centre. Through our history we’ve included Fabians, trades unionists, Methodists and Marxists, middle class and working class, city and town, young and old, liberal and communitarian.

Right now both the left and the centre wings of our party are concentrated in the cities, appealing to younger, more liberal and more middle-class voters – but drifting away from older, working class voters in towns. Brexit put that problem on steroids as leave voters in towns such as mine in Yorkshire felt neither Jeremy Corbyn nor Tony Blair were listening to them.

I’m worried too that some members from all sides of the party are criticising candidates for talking about patriotism. Of course we should be patriotic – if Labour can’t show people we love our country, we won’t ever get to be in government again.

Outgoing Labour leader Jeremy Corbyn in Blackpool during last year’s general election campaign. Photograph: Anthony Devlin/Getty Images

4 Learn to love the achievements of the last Labour government

If we keep slagging off the last Labour government, why are we surprised if voters don’t want another one? On the doorstep when we pointed out Tory failures, too often people responded “yeah, but was Labour any better?” Actually yes – GP appointments within two days not two weeks, fewer kids in poverty, more police officers, more buses, fewer people sleeping on the street, more investment in our towns … I could go on.

Of course Labour’s future programme needs to be more radical and ambitious for the 2020s and not go back to 1997. Of course there are things that the last Labour government got wrong. But we also need to show how Labour can make a difference.

5 Be a strong and credible opposition, as well as a radical alternative government

Boris Johnson’s government has to be held to account through forensic scrutiny – and that involves all of us, not just the new leader. Opposition is hard work but it matters. At stake is our NHS, our public services, the future of manufacturing industry. Britain is leaving the EU at the end of January and our future trading relationships will be crucial. We also have to be credible as an alternative government. It isn’t enough to admit the manifesto was too crowded and wasn’t believed if we still freak out when the leadership candidates tell us we can’t have every new policy we want. Socialism is still the language of priorities.

6 Bring kindness and integrity back into politics

The next leader must take strong action against antisemitism and they need all our support in doing so to stop racism or prejudice corroding our soul. But all of us have a responsibility to stop the abuse on and offline. I’ve seen those on both the left and centre of our party accused by other members of being traitors. We should leave that kind of vitriol behind and stand up for something better.

7 Get involved

Don’t despair. The first election I campaigned in was 1992 and afterwards I feared we would never win again. But I remember John Smith saying to young activists: “As long as people like you still join and fight for our party, I know we have a future.”

We are further behind than in 1992 and face bigger challenges, not least in Scotland. But we also still have great strengths and passion, amazing activists who worked hard for every vote and believe in a better Britain. And it feels like things change faster now so who knows what we can achieve if we try. So join. Or rejoin. Be part of this.

Those who built the Labour party more than a century ago created something incredible – a party for ordinary decent people who believe we must stand together when one of us is struggling. We won’t stop fighting for social justice. When we are strong, when we are broad, we change the world. That’s what we must do again.

• Yvette Cooper is the Labour MP for Normanton, Pontefract and Castleford