At its most successful, Ukip ostentatiously rejected the support of the far-right. Its current leadership's very public embrace of Tommy Robinson, the founder of the English Defence League (whose members are banned from joining Ukip), is a mark of just how far it has fallen.

Robinson – real name Stephen Yaxley Lennon – has been jailed for 13 months for contempt of court after streaming an hour-long video over Facebook from outside Leeds Crown Court. Gerard Batten, Ukip’s leader, is very angry indeed.

He has claimed the episode proves Britain is a “police state”, joined a protest in support of Robinson at the gates of No 10, and pushed calls for Sajid Javid to be privately prosecuted should Robinson be "murdered or injured in prison".

That Batten has thrown his lot in with Robinson is no surprise. Before succeeding Henry Bolton in February, the MEP was known for having called Islam a “death cult” and very little else. He shared a platform with Robinson at a notionally pro-free speech rally led by far-right figures, including the anti-Islam activist and one-time Ukip leadership contender Anne Marie Waters, earlier this month, and has touted for support among street movements like the Football Lads’ Alliance.

The party’s lurch towards alt-right territory makes the fags‘n’flags populism of Nigel Farage, who made a virtue of having wiped out the BNP and rejected overtures from Robinson and the EDL, look like milquetoast centrism. Even the explicitly nativist, anti-Islam pitch Paul Nuttall made at the general election was, just about, on the fringes of a recognisable mainstream – but where Ukip once spoke to mass-market sentiment along the lines of the Daily Express, it’s now much more InfoWars or Drudge Report.

Of all the questions this shift poses, two stand out: who cares, and why bother giving it any airtime?

Ukip is on an increasingly precipitous slide to political irrelevance: it won just three seats at the local elections earlier this month, having contested barely a quarter of the seats it did in 2014, and has polled more than five per cent just three times since last year’s general election, when it won 1.8 per cent of the vote. The electoral ceiling on the politics it is now championing is similarly low. Its decline looks terminal.

Why it matters, however, is that this need not have been the case. Even as Ukip's polling numbers collapsed in the wake of the EU referendum, it was arguably too soon to declare it definitively dead until this year. The calamitous leadership of Henry Bolton, the near-constant threat of bankruptcy, cataclysmic electoral losses and the looming departure of its 18 MEPs and their steady cash flow look set to finish it for good.

But were it not for its drastic retreat from the political mainstream – grimly illustrated by Batten’s embrace of Robinson this week – there might have been a way back. Though ineptly run, the party is very much a going concern. Some 20,000 members have yet to desert it and are still committed to its mission: an appeal to members raised £294,000 last month (it is doubtful that a large majority are comfortable with their party's direction of travel).

The narrative that the government is betraying Brexit has the potential to be almost endlessly lucrative, as the success of Jacob Rees-Mogg demonstrates. It is likely, at the very least, that Britain will still be in the customs union at the time of the next election. In constraining its politics so tightly and divisively, however, Ukip has surrendered its place in a seller’s market at the worst possible time.

Barring another drastic makeover, never again will it attract more than a fraction of the four million voters who backed the party in 2015 – and, as the Conservatives’ local election successes show, this will have lasting consequences for our electoral map and the tone and direction of both the government and the Tories on Brexit and beyond. Even as Batten rants on the fringes, Farage’s influence will endure.