An existential question confronts US progressives: how to win again in the age of Trump. It seems daunting, pitfalls haunting every possible move. But there is a path to revitalising your movements and, ultimately, power.

Nearly three years ago, British progressives faced similar challenges. In May 2015, the Labour party lost – and lost badly – an election it really should have won. The establishment narrative was that Labour’s mild resistance to soaring inequality and insecurity led to electoral oblivion. The party, so we were told, had to accept austerity, corporate control and anti-migrant and anti-social security rhetoric to confront those very evils.

Fortunately, Labour’s members, despite being told it would sink the party forever, ignored the established wisdom and elected veteran democratic socialist called Jeremy Corbyn. Corbyn, who Americans might see as the “UK Bernie Sanders”, ran an insurgent campaign and was elected on a wave of hope and optimism for a new kind of politics and a departure from business as usual.

That campaign developed into a movement called Momentum, which we co-founded with others from the campaign. We’re shaking up the political establishment and helping transform the Labour party to reconnect it with millions and put it on the path to victory, putting power in the hands of the many not the few.

In 2017, the ruling Conservatives called a surprise election from a position of apparently unquestionable dominance, 24 points ahead in the polls. They explicitly set out to bury Labour for a generation. But thanks to a massive grassroots effort, in part organised by Momentum, Jeremy Corbyn’s Labour party ran the most remarkable and record-breaking campaign in British political history. Labour increased its vote share by the largest amount since the second world war, gaining seats in Conservative strongholds and robbing the government of its majority in parliament.

It hasn’t been plain sailing. But under Jeremy’s leadership, the Labour party’s membership has tripled and it is one of the few left-of-centre parties in Europe making any sort of electoral gains, at a time when the US Democratic party struggles to define itself beyond opposition to Trump.

So we’d like to share three lessons we’ve learned about how to wrest control of a party away from technocrats and elites, and make it the party of the people that it always had the potential to be.

1. Don’t fight from the outside, take the movement into the party

Before Corbyn’s unlikely leadership, there was a sense that the Labour party had lost touch with its heart and soul. Despite growing out of the unions, the largest social movement in our history, relations between movements and the Labour establishment had cooled in recent decades.

Nothing demonstrates the limitations of an “outsider only” strategy more than 1 million people marching on the streets of London in 2003 (including Jeremy Corbyn, who helped organise and spoke at the demonstration), while the Labour government, then led by Tony Blair, ignored them and launched us into a disastrous war in Iraq.

The last two and a half years have taught us the power of an insider-outsider strategy, linking movements and the party to create a movement-party.

Corbyn’s leadership opened the door to activists from a wide variety of movements: from peace to environment to racial equality or anti-austerity. These activists and the many more inspired to join are bringing their energy, experience and creativity to the party.

From the Women’s March to the airport protests against the Muslim ban, the ongoing work of Black Lives Matter, and the current mass protests led by high-schoolers for gun control, there has been an explosion of mobilisation in the US in the Trump era. If the Democrats are going to take back power, the party has to do more than listen to these people: it has to show them they are committed to making changes equal to their demands.

And the movements must engage with the Democrats and, if necessary, force the party to hear them. Now is the time for movement people to get involved in the Democratic party, at every level, and elect people into positions of power and influence.

One great example of the movement-party, or insider-outsider, strategy we’ve seen in the US is the Real Justice PAC to elect reform-minded prosecutors in county and district attorney races.

Don’t leave it to someone else. You are someone else.

2. It’s politics, stupid

Since the election, many have asked us what social media or digital tools we used to mobilise so many people. Unquestionably, Momentum’s social media and digital team did a fantastic job (actually just a handful of extremely hard-working, talented and – regrettably – underpaid people, and a top team of volunteers).

A staggering one in three UK Facebook users saw a Momentum video during the general election – almost entirely through organic, unpaid reach. But we think that these questions miss the point.

People young and old weren’t mobilised in their tens of thousands to knock on doors for the first time by clever digital tools. We wanted to get involved because for the first time in many of our lifetimes, we were being offered a positive and hopeful vision for a more equal and decent future for our country.

What galvanised people were the promises of a £10 per hour minimum wage, 1m new homes to be built to end our housing crisis, free university tuition, tax rises for the top 5% and none for the rest of us, and a host of other concrete policies that offered solutions that are as big as the problems we face.

So, don’t kid yourselves by thinking there is a magic bullet that will swing that critical mass of voters. You can’t solve big problems with incremental, technical policy solutions. Just saying “we are not Trump” or advocating a return to the status quo won’t cut it.

In 2015, Labour ran with the message that they would “cut less” and be a little nicer than the Conservatives – and they lost badly. In 2017, we ran on a bold platform offering big solutions to peoples’ real problems – and our vote went up 10%.

It has to be a big political offer that will materially improve the lives of the many. That’s what the Democratic party has to be encouraged to put forward at the next election – so become the Democratic party and make sure that’s what it puts forward.

3. Make it fun, make it exciting, make it diverse

For the project to succeed, it needs to be engaging, exciting and fun for as wide a group of people as possible. Gone are the days of politics being left to the “experts” and the political class. From Brexit to Trump, the anti-establishment sentiment and disillusionment with business-as-usual politics needs to be listened to in a sincere way. It’s easy to blame the right for a toxic political culture, but with a bit of self-reflection, there is a lot more the left could do, too.

So alongside the concrete political offer, there needs to be a radical rethink of the culture and practice of how we do politics. This can often be dismissed as “fluffy” or less important, but in our experience, it has been fundamental to the strategy of making political change.

It’s pretty straight forward, really. To build a mass movement, it has to be appealing and welcoming to people who have never been involved before (ie the vast majority of the population). And these people need to be able to shape and direct the project at every level.

Would someone who just walked in off the street enjoy your political event? Would they understand what was going on and feel able to participate? If not, what needs to change to make that happen? Are the people involved reflective of the community at large – or is it too male? Too white? Too affluent? Too old?

Why might that be and what could be done to change it? There’s no one-size-fits-all approach here, but everyone should be asking these questions in a meaningful way and sharing what’s worked and why.

There should also be the expectation on anyone in a position of leadership within the movement to use their position to enable and empower as many other people as possible. It is in the small acts of encouragement, mutual support and mentoring, that our politics are put into practice.

A great example of this in the UK, is The World Transformed, Momentum’s four-day festival of politics, arts and culture that has run alongside the official Labour party conference for the last two years. Thousands of people attended more than 200 hours of talks, workshops and parties, and even our biggest political opponents had to admit it was something special.

What makes that even better is that the whole thing has been organised by an incredible group of people (mostly volunteers) in their teens and 20s, which really shows the power of allowing people new to politics to shape the project.

The conditions in the US are ripe for a wholesale revival of the Democrats as a true party of the people. But it’s going to take everyone to make it happen.

• Adam Klug and Emma Rees were co-founders of Momentum.