One of the few things that worries Jeremy Corbyn’s inner circle is the prospect of a new party. Although they don’t believe that it would defeat them, they do believe that it would deprive Labour of enough votes to keep them out of office.

Are they right? Given that the shape (or even the possible existence) of a Labour breakaway is so unknown that it’s a classic “how long is a piece of string?” question. A great deal has changed in the United Kingdom since the SDP/Liberal Alliance got 25 per cent of the vote but just 23 seats, yet that party took votes about equally from the two major political parties. Over the long term, the Liberal Democrats became more of a handicap to Conservative aspirations of a decent parliamentary majority than they did to Labour.

My instinct is that it is the Conservatives, rather than Labour, who have the most to fear from any change in the political ecosystem, simply because it is Conservative voters who are the most underserved by the current menu of parties on offer. If you want something more centrist than Labour but still recognisably on the centre-left, then there’s the Liberal Democrats. If you want a party that is more radical on drugs, the environment, and crime, then there’s the Greens. And the SNP and Plaid Cymru are also plying their wares in that left-cosmopolitan space. But there is a much thinner list of options on the right than the left at present.

But that’s just a hunch and so it’s not worth very much. A more interesting question is what it does to the existing parties. As far as Labour go, the immediate effect would be to achieve overnight what the leader’s office may not be able to achieve at all: replacing a swathe of Corbynsceptic MPs with loyalists. Although the leader’s office is not having its own way in parliamentary selections, with many preferred candidates struggling, the majority of those selected backed Corbyn in the 2016 leadership election even if they are more politically diverse than that would necessarily lend you to expect. There is no reason to believe that selecting candidates in seats left vacant by defection would be any different, especially as any defector would hope to take at least some loyal activists with them.

It would also add to the mood music around Corbynsceptics in the Labour party that they are cuckoos in the nest. Whether that would be cancelled out by the effect of removing some of Jeremy Corbyn’s more tactically maladroit critics (one Corbynsceptic Labour MP recently told me that they would never retake the party “until Chris [Leslie] fucks off”) is an interesting, but unknowable variable.

What would it do to the Conservative Party? Well, again, it would have the result of making the party they had departed more inimical to their interests. On balance I think this is a smaller risk. If you look at the long term trend, new intake Conservative MPs have been getting more hostile to Europe essentially regardless of where they sit on the party’s left-right divides, whereas there is little to suggest at the moment that Corbynism is a movement more capable of permanently reshaping the Labour party rather than just electing Jeremy Corbyn as its leader.

But in both cases it underlines the big risk MPs are taking if they do split off: which is that by leaving, they make the party they used to be part of more hostile to their politics, and if a split ends in SDP-style failure it may be their politics, not that of Corbyn or May, that goes up in smoke.