A Final Say referendum is the only thing standing between Corbyn and victory at the next general election

The last few weeks have radically changed the face of British politics. The Liberal Democrats, having languished in single figures for the best part of a decade, now vie on equal terms with the other parties in what appears to be a new kind of four-party politics. Chuka Umunna’s decision to join the Lib Dems last week raises the prospect of others following. The two traditionally major parties are cracking and melting like the Arctic ice sheet.

While the media inevitably focus on the uncertainty around who will lead the Conservative Party, there are three potentially more important imponderables of a structural rather than personal nature.

The first is whether a Brexit deal is deliverable under a new Tory leader. Merely to ask the question is to perpetuate a myth which dogged, and eventually destroyed Theresa May: that the British – through negotiating skill or threats – can change the terms of Brexit. The reality is that the 27 members of the EU are not going to offer the UK a better deal outside the EU than inside it, just because a new PM bangs the table, raises his voice and speaks slowly.

In truth, none of the Conservative leadership candidates offer any new thinking about the main stumbling block in the withdrawal agreement: the Irish backstop. The “max fac” technological solution favoured by Boris Johnson has already been considered and rejected as unworkable.

A possible, but not very plausible, route to delivering a deal is that Johnson pleads for some minor, synthetic changes to the “comfort letter” from the EU which was previously declared to have no legal force. Then, having appointed a plausible minion as attorney general, the new PM would get him or her to endorse it, where a lawyer of integrity like Geoffrey Cox had refused.

However, such cynical manoeuvring would still then require the DUP, wavering Tories and perhaps 20 to 30 Labour Leavers to suspend their critical faculties to “get Brexit over the line”. It could happen, but it is somewhat far-fetched.

A second question is: if a new deal is not negotiable, or is rejected, is “no deal” then inevitable or, at least, likely? Notwithstanding the rejection of Oliver Letwin’s procedural motion last week, I have never sensed that there is any willingness, except on the fanatical fringes, to allow no deal to happen by accident or design.

Parliament is overwhelmingly opposed and has a variety of options still available to stop it. A no-confidence motion leading to an election (which prominent Conservatives have said they will be supporting in extremis); the adoption of a Final Say referendum (which is winning some reluctant support even from the likes of Ken Clarke, if no deal is the alternative); or, as a last resort, revocation of Article 50.

Brexit cannot be achieved without parliament passing a raft of legislation – on migration for example and a trade bill – all of which provide opportunities for the Commons to steer a new course. And the European Union has made it clear that Brexit could be delayed, again, if there were the prospect of an election or referendum.

My third question is whether Jeremy Corbyn could win an election. This question matters because the main factor inhibiting many Conservatives (and some Labour supporters) from precipitating a general election through a no-confidence vote is the fear that Corbyn could win.

His unexpectedly good performance in 2017 helps to keep this nightmare vivid. In practice, it is difficult to see Labour making headway under Corbyn, let alone winning. Scotland has abandoned Labour. Some north of England Leave seats will probably go Conservative or to the Brexit Party, and some heavily Remain, London and university seats held by Labour would probably fall to the Lib Dems.

It is possible that, with the Conservative vote splitting off to the Brexit Party, large numbers of Tory seats could switch to my party too. If that happened, the Lib Dems could then plausibly be a stronger force than either or both of the “big two”.

It is just about possible to see a combination of chance factors helping Labour to edge the Conservatives for the largest number of seats in the Commons, but they would be well shy of a majority. Fear, then, of his enacting a ruinous socialist agenda is misplaced. A Corbyn socialist government is, in the current jargon, a “unicorn”. Corbyn’s real thinking is that a hard Brexit followed by hard economic times might be the only thing which could make the unicorn real.

We may not get clarification on these issues until October and it promises to be a climactic month (but then, we said that about March and May). For those troubled by uncertainty, hold on tight, you are in for a rough ride.

Sir Vince Cable is the leader of the Liberal Democrats