Look, I get it. The headlines wrote themselves last week as more Change UK debris reformed as the Independents. Which faction was definitely, absolutely, fighting on as a party, and which bit was refusing to establish one? Was this the Peoples’ Front of Judea, or the Judean Peoples’ Front? Who got custody of Sarah Wollaston?

It feels like some time ago, but for one short period back in the spring, it looked as though the formation of the Independent Group might just catalyse the unravelling of both parties of government. As one of the key instigators of the plan, I can tell you there was only one real metric of success that first week: could we get to the end of it without becoming a punchline? That we pulled that off was a testament to the desire of millions of people, feeling as politically homeless as the MPs that jumped, for something hopeful.

After last week’s revelations, you know everything you now need to about your leader

We all know what happened next. The formation of the political party, Change UK, and its subsequent rather small and imperfectly formed electoral showing meant that moment passed almost as soon as it came.

But take a step back and honestly assess the bigger picture: huge political forces, surging through a parliament that cannot meet the enormity of the challenge it faces, are still present. A Labour leadership that is incapable of dealing with antisemitism, propped up by MPs who hope the British public will do what they will not, and prevent them from getting into No 10. A Tory party that will select a prime minister who will tank their reputation for economic competence in a bid to deliver Brexit. The fracturing of electoral coalitions, ages old, which has led to four parties (including one that did not exist even six months ago) bobbing around in the polling doldrums at around 20% each.

Nothing can be done, the moderately minded MPs of both legacy parties say, look at what Change UK did and take heed. Our failure is used as the proof point that something new can’t emerge. And yet: in many respects, the case for a proper realignment is stronger than when we left our parties.

I shoulder my share of the blame for soiling the pitch. To rush to establish a political party, as just 11 MPs, was an act of hubris. That logo! The bus! We took the kicking that we deserved, especially when our desire to stand in the EU elections seemed destined to split the remain vote. But to use our (predictably) poor showing as an excuse for inaction, is surely the most cynical response of all.

We were never going to be able to provide a compelling, centre-ground, future-focused electoral offer on our own. The act of leaving our parties was the beginning of something, not an end in itself. But others still need to face up to their own dilemma in representing parties they believe are unfit for office.

So – to the real point of this missive. To my former Labour colleagues (for the Tories’ moment of reckoning will come in the autumn) let me say this: Don’t let Change UK put you off from acting.

After last week’s revelations, you know everything you now need to about your leader. He isn’t going to change: the shutting down of democratic processes and debate; his instincts on national security; the vacuum of leadership on Brexit; the stain of anti-Jewish racism in the party and his lack of interest in getting a grip. Truthfully, all this was visible the last time you passed a vote of no confidence in his leadership by a margin of four-to-one.

Only two things have changed: his grip on the machinery of the party (insurmountably tighter) and his proximity to Downing Street (undeniably closer). The final foxhole that the moderate Labour MP will sink themselves into is “he’s unelectable, so it doesn’t matter”. Even if this were the case, it should matter. But I think that assessment is also catastrophically wrong. This was the prevailing wisdom last time around, before Labour went on to gain seats and deny the Tories a majority.

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If all you can come up with is stay and fight, you’re actually going to need to do both. In 2016, the Parliamentary Labour party delivered 172 votes to challenge Jeremy Corbyn’s leadership. Today’s threshold is 52 MPs and MEPs, and the window for a challenge is closing. If you don’t act now, you will be heading for a speculative autumn election, and campaigning to make a man you believe is unfit to lead your party the leader of our country.

There are just 10 days to go before parliament slumps into its long summer – if you can get one in five of your colleagues to support a challenge to the leader of the opposition, you must act now. And if you can’t clear even that low bar, then at least be honest with yourself – the fight is lost. All you’re doing is staying and propping him up.

History is not just shaped by action, but by inaction as well.

• Gavin Shuker is the MP for Luton South and a member of the Independents

• This article was amended on 15 July 2019 to clarify that the writer is arguing that antisemitism is an issue in the Labour party rather than the Labour leadership