Douglas Carswell was always a very atypical UKIP politician. This morning’s Observer reports that Leave.EU have branded him a ‘Tory plant’ and whilst that may not be quite true, by his own admission his motivations for joining up resemble those of an entryist.

According to Tim Shipman’s seminal book on the EU referendum, All Out War, his explicit goal was to go in and try to ‘detoxify’ it before the contest began in earnest. He may also have planned long-term to make sure that a moderate campaign had high-profile UKIP support in the event of a contested nomination.

So his abandoning the People’s Army now, when his stated reasons for ever having joined it are complete, need not be a signal that the party is in terminal decline. But as John Rentoul points out in the Independent on Sunday, there are plenty more:

“The referendum deprived it of its reason for being. Farage’s last resignation (the third or fourth, depending on your definition) thereafter deprived it of its most charismatic personality. His successor Paul Nuttall’s defeat in the Stoke by-election last month ended its chance of being the anti-Labour protest party of the North and Midlands.”

Having lost its only MP (and thus any hope of Short Money), with its contingent of MEPs set to lose their jobs in a couple of years, and having parted ways with Arron Banks, UKIP’s days as an operationally-viable party may be numbered. If this is the case – and it may not, two years being a very long time in politics – it will serve history as a fine example of the British party system doing its job.

Our system makes it very difficult for minor parties to break through, except where they have geographically-concentrated support. When one finds a real current of popular sentiment and starts to exert great pressure on the major parties, one or both of them adapt and absorb the popular parts of its agenda. They often strip out some of the more noxious or radical elements for good measure.

Brexit is a case in point: it’s not difficult to imagine UKIP doing better if Theresa May hadn’t moved the Conservatives so decisively onto its political territory. Scarcely anybody at this point can believe that the Prime Minister is not deadly serious about delivering a real/hard/clean break from our membership of the EU. Brexit doesn’t need a party.

Meanwhile the Liberal Democrats are enjoying a resurgence precisely because their political space has been unexpected vacated. Whilst the Conservatives have moved right, Labour under Jeremy Corbyn have wandered off the leftward edge of the map altogether.

‘Metropolitan liberals’, or whatever you want to call them, are finding a sharper political identity in the aftermath of their referendum defeat. They used to more or less run all three of the main parties; now it increasingly looks as if they’ll need one of their own.

Will this lead Tim Farron’s party to the sort of major third-party breakthrough that Sir Nigel Farage’s never managed? Perhaps not: a very good result for them looks right now to mean 20-something seats, about a third of their heyday. It’s also important not to overstate the Richmond by-election, where they defeated an MP who lacked party support, association data, or the advantages Tories will possess when the voters are actually choosing a government, by a fairly narrow margin.

On the other hand, Corbyn’s leadership may have short-circuited the usual two-party immune system. If Labour can’t move onto that liberal ground – and there are serious strategic challenges to their doing so – then the Liberal Democrats (or some new party built out of them) may build up sufficient pressure to break through. Major parties do fail in the British system: the Liberals did a hundred years ago, and it could be Labour’s turn. Sometimes a political era passes, and its party with it.

It may be UKIP’s misfortune, as an institution, to be competing with the Conservative and Unionist Party, one of the most formidable political survivors in history. But it is surely Euroscepticism’s great gain: by the time ‘Continuity Remain’ have been through their realignment, their moment may have passed as well.