At Westminster, MPs are quitting their parties out of disillusionment with the political directions they have taken.

But in Cardiff, a council by-election victory has been engineered by an AM who has been expelled by the party that won.

While Andrea Gibson is the new Plaid Cymru councillor, she would not have won without the help of her de facto campaign manager Neil McEvoy.

For Labour and, to a large extent, Plaid, Mr McEvoy is unwanted.

He often provokes jeers of disdain from his opponents in the Senedd and the council chamber when he makes an angry intervention.

But for many on his home patch, he takes their concerns seriously in a way they believe other politicians do not.

In March last year, Mr McEvoy was expelled from Plaid for 18 months for breaching a series of party rules. The judgement referred to his behaviour at the party's spring conference in 2017, soon after a panel found him guilty of making a bullying remark to a council official involved in evicting a female council tenant from her home.

Gareth Clubb, the party’s chief executive, also complained it was wrong for Mr McEvoy to call a press conference about other complaints that were made against him.

Later the South Wales Central AM’s period of expulsion from Plaid was reduced to one year, which means that in less than a month’s time Mr McEvoy will be able to apply for readmission.

(Image: Neil McEvoy)

There is no doubt that his enemies in Plaid will try to block his return.

Before he was expelled from the party, he had been booted out from Plaid’s Assembly group, with its members saying there had been an irretrievable breakdown in trust.

What happened in Ely on Thursday is much more than an unexpected by-election result that will follow the usual pattern of being a nine-day wonder.

Plaid’s victory occurred in the Cardiff West seat held by First Minister Mark Drakeford, who was run close by Mr McEvoy at the last Assembly election in 2016.

With Mr McEvoy as Plaid’s candidate in 2021, the party would have a real chance of ousting the leader of Welsh Labour and getting into a position where its leader Adam Price could be elected First Minister – an outcome that would revolutionise Welsh politics.

While Ms Gibson undoubtedly worked hard for her victory, it was Mr McEvoy’s strategy that propelled her into the council chamber.

He is a highly astute user of social media, and during the campaign made countless videos showing empty council homes in Ely with gardens full of rubbish.

It’s easy to parody his style, which invariably sees him beckoning the viewer to follow him into a waste-strewn area and watch as the camera pans while Mr McEvoy rants about the shocking indifference of Cardiff’s Labour-controlled council.

Yet his invective is effective. It tells residents of a neglected neighbourhood that in him they have a champion. On Thursday enough of them were sufficiently enthused by his message to vote for Ms Gibson, his protégé.

After the result senior Plaid AM Rhun ap Iorwerth tweeted: “Congratulations Cllr Andrea Gibson! Llongyfarchiadau [Congratulations). This is Ely. This is Cardiff. This is @Plaid_Cymru.”

Former Labour councillor Siobhan tweeted back: “I think it’s more @neiljmcevoy tbh.”

The unexpected council by-election result is a challenge both to Plaid Cymru and to Labour.

Labour needs to ask itself why it lost the seat despite throwing everything into its campaign.

But Plaid must decide whether to take Mr McEvoy back into its tent. There will be many who don’t want to do so. But they will have to consider the consequences.

Can they afford to throw away the opportunity to defeat the First Minister? No-one else would have a chance of winning Cardiff West against Mark Drakeford.

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The Ely campaign shows that adopting a populist approach can be electorally successful for Plaid in places where they normally wouldn’t stand a chance.

All Plaid needs to do to be in a position to form a minority Welsh Government in two years time is win a few seats and - with the support of the other parties - Adam Price could be elected First Minister in the first meeting of the new Assembly term. Neil McEvoy could be the kingmaker.