Brexiters stop at nothing to get what they want and remainers stop at everything. The laws of political motion then dictate which direction things move.

Jeremy Corbyn has written to MPs inviting them to install him in Downing Street, having deposed Boris Johnson with a vote of no confidence. His tenure would, he promises, be “strictly time-limited” – long enough to call a general election and seek the necessary article 50 extension to conduct a ballot.

For Corbyn this is the simplest route out of the current mess. There is a government hell-bent on doing something that a majority of MPs oppose and believe to be ruinous – hurtling off a Brexit cliff-edge. The Fixed-term Parliaments Act gives the Commons 14 days to organise a replacement when an incumbent government is defeated in a no-confidence vote. Who else is going to lead that administration if not the leader of Her Majesty’s opposition? In constitutional terms he is the obvious candidate; probably the only candidate.

But in the minds of scores of MPs he is not. His past equivocations over Europe are not the reason, or at least not the only reason. Pro-European Tory rebels, Liberal Democrats, the rag-tag platoon of independents and semi-autonomous tribes of Labour MPs have spent months fretting about ways to thwart a hard Brexit, apparently ready to pull every procedural lever and contemplate all manner of unorthodox coalitions. Not much has been excluded from those considerations, except for a tacit prohibition on any route that makes a prime minister of the current Labour leader. Their horror of Corbyn is equal to – or greater than – their horror of Brexit. That has been so well understood by the participants in the discussion that few have felt much need to articulate it. Corbyn’s letter now obliges them to spell it out.

That is easier for some than others. Tories and ex-Labour independents don’t have much difficulty vocalising reasons why they think the Labour leader is unfit for office, even on a time-limited basis. They see him as a political extremist, a friend of terrorists, Putin stooges and antisemites. They think he would bring to Downing Street a sinister creed and a cabal of advisers whose very presence inside No 10 would sabotage the safety of the UK. For MPs who feel that way, the objection to a single day of Corbyn rule, even for a tactical purpose, is visceral and moral.

But there are others – Greens, Lib Dems and Labour moderates – who, if they share that passionate aversion, are reluctant to express it in public. Jo Swinson comes close. When elected to lead the Lib Dems she ruled out an alliance with Corbyn on the grounds that he couldn’t be trusted on the European question, but also (added almost as an afterthought) because “he is dangerous for our national security and for our economic security, too”.

Jeremy Corbyn urges MPs to back Labour plan to block no-deal Brexit – video

Swinson dismissed Corbyn’s letter swiftly last night, largely on the basis that others would surely do so too. The Lib Dem leader judges that Tory rebels and independents will not get on board and so the parliamentary numbers simply aren’t high enough to express confidence in a Corbyn-led administration. That is arithmetically correct, although it is also a circular argument. The numbers don’t add up because a number of MPs say they will not add themselves to the number. What is missing from the equation is the courage to explain what it is about Corbyn’s politics, character, judgment and ethics that makes those MPs subtract themselves from this total.

That is hardest of all for Labour pro-Europeans, for the obvious reason that Corbyn is technically their leader. Many of them fervently wish he wasn’t, but also hope that some external factor will remove him without them having to advertise their dissent. They fear the backlash from party members, the majority of whom are still committed to the man they elected twice as their tribune. Many of the Labour moderates have been counting on Corbyn’s instinctive Euroscepticism and cavilling over an EU referendum to alienate the party faithful. But he has filibustered shrewdly, avoiding total commitment to the remain cause but without furnishing his internal Europhile critics with a moment of distinct betrayal. And now he is saying Labour would fight a general election promising a referendum with remain on the ballot paper. That is what the pro-European MPs in his party have been asking for but not all that they really want.

There is something disingenuous about the discussions among MPs about a “government of national unity”(GNU) to avert a no-deal Brexit. It is predicated on concepts of nation and unity that don’t include those who are desperate to leave the EU. Those who voted leave are broadly satisfied with the government they currently have. It is, in truth, a euphemism for a model of technocratic, centre-facing liberal administration defined as much by a rejection of Corbynism as by revulsion at the Trumpian nationalist character that Brexit has acquired. There are many voters who would be glad to have a moderate, bipartisan government. They can play fantasy football with exotic cabinet combinations – Dominic Grieve as chancellor; Keir Starmer to fix Brexit; Caroline Lucas for the environment; Jo Swinson for home secretary. And the prime minister? David Lidington? Yvette Cooper? Anyone as long as it isn’t a Johnson … or a Corbyn. But the Commons numbers don’t add up for that either, unless most of the parliamentary Labour party abandons the whip. It won’t.

The Labour leader knows this and he is calling the whole GNU bluff. If a government falls, the opposition leader is the next in line to have a go and, if that can’t be arranged, there is an election. That is how it works. There might be many reasons why MPs do not want an opposition leader to take charge – that is their constitutional right, too – reasons of tactical political advantage and reasons of conscience. But MPs have not all been candid about what those reasons are; why it is that so many find Corbyn as toxic as Brexit. Their problem is that there aren’t a lot of other options. And the laws of political motion are working against them.

Rafael Behr is a Guardian columnist