After living in Germany for over 20 years, I joined the Social Democrats (the SPD) in early 2017. Martin Schulz, the party leader, had just taken the unprecedented step of criticising the infamous “Agenda 2010” welfare cuts of the last SPD chancellor, Gerhard Schröder.

The Agenda, as it is known, is to the SPD what Iraq was to New Labour. In squeezing pensions, slashing benefits and pushing millions into low-paid, precarious employment, the SPD abandoned its electorate. The electorate responded in kind: since Schröder’s first election victory, in 1998, the party has lost more than half its support – plummeting from more than 20 million voters to 9.5 million in 2017. Most of all, it cemented the SPD’s reputation as a party of pushovers, too open to the siren overtures of lobbyists: indicating left in elections, turning right in office.

I joined the SPD in spite of some sympathies for the more radical Die Linke. As a member of the overseas section of the British Labour party, I’d been warned that support for anyone but the SPD (Labour’s official sister party in Germany) could lead to my suspension from Labour.

The NoGroKo campaign is an attempt to rejuvenate a decaying social democratic party, rather than start a new one.

But, inspired by what has happened under Jeremy Corbyn, I am also convinced that the SPD has the potential, if not yet the will, to completely reframe the political agenda. If the SPD finds the ambition to end the gaping injustice in German society, it can flip the debate in a way in which Die Linke has never been able to – but just as Corbyn and Momentum have done in Britain.

Schulz’s rejection of the Agenda was thus a signal moment. Maybe the SPD could regain some of its credibility and vision – and even open up to a coalition with the Greens and the more radical Linke. Briefly, the SPD soared in the polls.

These hopes were dashed. Surrounded by a party establishment still intimately associated with the Agenda, Schulz back-pedalled, – and the SPD crashed to a humiliating 20.5% of the vote on election night last September: its worst postwar result.

The only comfort was Schulz’s impromptu vow not to enter another grand coalition. It saved him his job. And for a party membership still longing for the long-lost moral standing of its legendary leader Willy Brandt, it finally seemed as if the SPD could finally rediscover its core values in opposition, instead of propping up a centre-right government under Merkel yet again.

But when coalition talks between Merkel’s Christian Democrats, the Greens and the free-market FDP unexpectedly broke down last November, Schulz’s vow was broken overnight. The SPD establishment skidded into a hamfisted U-turn – and the grassroots is up in arms.

There will now be a membership ballot on any grand coalition agreement – and there’s a real chance the leadership could lose. Last Sunday’s party conference – normally a tightly stage-managed affair – saw unprecedented dissent, and our NoGroKo (no grand coalition) campaign is mushrooming on social media. Thousands have joined the SPD in the last few days to vote.

As a member of both Labour and Momentum, this a dynamic I have seen before, at first hand. Here, finally, is the chance for a genuine, credible renewal of the party: not from the top down, but the bottom up.

First, it’s a chance to reject a neoliberal consensus that is leading us into disaster. The 45 richest Germans now own as much as the 40 million poorest. This obscene figure will only get worse in a grand coalition.

Second, it’s a chance for democracy to flourish in the SPD. In a party where the leader and six vice-chairs are traditionally elected unopposed at conference in a spectacle worthy of Pyongyang, the membership ballot is a unique opportunity for the grassroots to reassert control.

The party establishment is painting a doomsday scenario of new elections in which the SPD is wiped out. The argument is weak: fresh elections are inevitable only if Merkel turns down the opportunity to form a minority government with the Greens. And an SPD wipeout would be inevitable only under the current leadership. Schulz’s criticism of the Agenda in 2017 gave him a 10-point boost in the polls. This time, for such a stance to be credible after so much flip-flopping, it needs to be anchored not in the character of any new leader, but in the democratic control of the membership.

The parallels to Corbyn and Momentum are striking. Within the SPD as within Labour, the struggle pitches a grassroots longing for change against an establishment offering more of the same. In a dynamic redolent of the Corbyn leadership campaigns, thousands have joined the SPD since Sunday to vote in the ballot.

Similarly, the NoGroKo campaign is an attempt to rejuvenate a decaying social democratic party, rather than start a new one. It has inspired an alliance between the young, older members, returning members and the broader left. As with Labour, it’s this alliance that can credibly renew the party from below.

Most of all, Corbyn and Momentum’s success in completely reframing the British political narrative gives us the most precious resource: hope. Our greatest obstacle is not the SPD leadership, but rather the resignation of those do not yet believe that meaningful change is possible. Momentum provides the blueprint, vision and skills – but more than that, proof of concept.

The assertion by some in the SPD establishment that a Corbyn-style shift would be impossible for the SPD in the German proportional representation system betrays how badly they misunderstand the Corbyn phenomena. Supporters did not flock to Corbyn’s Labour because of his personality – but rather because he credibly represented an unflinching set of values.

There is so much at stake for the German left. Each grand coalition strengthens the far-right Alternative für Deutschland. With 94 MPs the AfD is now third-largest parliamentary group – and in a renewed grand coalition, will become the leading opposition party, with an increased profile in every debate – and every TV talk show.

Once again, the spiralling economic and political alienation fostered by neoliberalism and its short-sighted centrist advocates has created the breeding ground for the politics of hate. With coalition talks under way, the SPD’s poll ratings have now slumped to just 17% – and the AfD are hot on their heels at 13%.

All over Europe, formerly socialist political parties are collapsing to single figures. It’s more than possible that one more grand coalition will see the SPD wither and die – and the AfD replace it as Germany’s second party.

It’s a terrifying prospect. But it’s not the only one. With a bit of luck, this could be our Momentum moment.

• Steve Hudson worked in the Momentum social media and video team in the 2017 general election and is the chair of NoGroKo