After the Brexit vote in 2016, it might have seemed that we would never need to argue for Britain’s role in Europe ever again. Now that the mood has changed, it is time to make that case once more.

For progressives, it is crucial we get this right. Just saying that “nobody voted to be poor” or “we will be worse off out of Europe” is not enough when people are already suffering in poverty at the hands of the Tories. There is a much stronger argument here: our principles.

So how to argue for our European membership from a point of principle? Here is my advice.

Don’t start with the word European. Start with the word union. What is it to be part of a union? The answer is simple: solidarity. It means recognising that there are just causes that should be fought for that are not for your own immediate benefit. If ordinary people club together, they are more powerful than if they fight alone.

In 1993, when the European Community became the European Union, perhaps it was this simple point of collective strength that they had in mind. As former European Commission president Jacques Delors said to the Trade Union Congress in 1988, “Thanks to cooperation and solidarity between Europeans, we will succeed in preserving our identity and our culture. Through the richness of our diversity and our talents, we will increase our capacity for decision and action.”

The European Union brings solidarity across borders, and turns our principles into action.

You don’t have to think that every trade union official in British history was a perfect human being to believe in trade unions and the principle of solidarity. And you don’t have to believe that the EU is perfect to think that in principle, it’s right that countries stand together for democracy and justice, against dictators and despots who would return us to years of conflict.

In fact, where Europe has fallen short is when solidarity has been undermined or where our politics has been unequal to the values that, at our best, we act on. Be it the treatment of migrants, or the approach to economic solidarity, the politics is imperfect. The idea that we stand together is not. But it is an argument in favour of the EU that must be made now because of the threats we face. If you worry that globalisation needs political masters that can set the rules of the market in favour of those with least – the average person as worker or consumer – then the European Union, imperfect thought it is, is the best means we have for achieving that goal on a scale big enough for the challenge. The UK going it alone won’t work.

In principle, and broadly in practice, the EU has been a progressive force in European society.

Take the most controversial issue in the UK: free movement. Some on the left (and the right, and in the centre) have spent the past few years rightly worrying about wages, wondering whether an end to free movement brought about by Brexit might help deal with Britain’s stagnant incomes. It is a reasonable question.

But what is completely unreasonable is forgetting that, whatever the downsides to be tackled, free movement itself is part of the union deal that offers us all something in exchange for something else. In our fight against poverty pay, we have forgotten to fight for the coming generation to have the chance to learn and work anywhere in Europe. People think that is a middle-class concern, but that’s rubbish. Tell that to the working-class young person who does a season doing nails in Ibiza. Tell that to the person from a poor background who goes to university and spends a term in Holland or Germany.

You can pretend that British people don’t want to be free to travel around Europe like everyone else, but the numbers of people queueing at Manchester, Leeds Bradford, and Luton airports will show this lie for what it is. Freedom to move about is a thing that working class people want just like anyone else. What’s more, there is a practical side to the free movement deal that Britain needs. We constantly forget that our country is, like most developed nations, ageing. Our working population is shrinking relative to our elderly population. We need an answer.

Women don’t want to go back to being unpaid carers or start having a great deal more children. Part of the deal of the EU is that countries like ours are able to boost their labour market, while countries that have younger people prepared to move for opportunities can boost their income. We have forgotten this side of the deal. We have allowed free movement – which is a good deal – to be cast aside, because immigration has been used as an excuse by conservatives and extreme free marketeers to justify poverty pay and the re-emergence of hate in British politics.

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That is a mistake, in my view. It’s time to get back to campaigning for the union – and the principles – we believe in.

And one final point about solidarity. To those who say these arguments risk the rise of the far right, I would ask them to think again. Fascism does well where people’s basic needs – a home to live in, affordable food, a job with dignity – are denied. As they are being denied now.

The principle of solidarity between people, as well as across and within borders, is far from offensive to most people. The actual risk to us all comes from poverty and our current predicament being without hope. Unions build strength, not weakness, and I cannot for the life of me think why we wouldn’t keep arguing that now.