In the parliamentary drama of the coming week, the term “Namierite” is one you are unlikely to hear often, if at all. But it matches perfectly this moment in the Brexit saga – wretchedly so, in fact. Sir Lewis Namier (1888-1960), one of the great historians of the 18th century, argued, on the basis of detailed forensic analysis, that most political behaviour is not motivated by high idealism, but the complex interaction of faction and connections, the search for personal advantage and the force of self-interest.

In his masterpiece, The Structure of Politics at the Accession of George III, he wrote: “Men … no more dreamt of a seat in the House in order to benefit humanity than a child dreams of a birthday cake that others may eat it … the seat in the House was not their ultimate goal but a means to ulterior aims.”

Which might strike you as horribly cynical – until you look at the present conduct of MPs as they prepare for Tuesday’s “meaningful vote” on Theresa May’s Brexit deal (the second, of course, following her thumping defeat first time round). It is a panorama of snivelling self-interest, squirming caucuses and abject tacticians posturing as statesmen.

Please bear in mind – because it is easy to forget in the political quagmire – what is at stake this week: nothing less than the constitutional, economic and social future of this country. So why are Tories even talking about their party’s leadership – as though the date of the prime minister’s departure will have any impact upon, say, the vexed issue of the Irish border, or the risk that the NHS might run out of life-saving medication, or that certain foods could become unavailable?

I have been arguing that May should quit since she squandered the Conservative majority in the 2017 general election. But – with fewer than 20 days to go until the UK’s official date of departure from the EU – of what possible interest is it that Amber Rudd may now (reportedly) be thinking of backing Boris Johnson, or that Esther McVey is preparing a bid for the top job? Have these people – or their surrogates – looked up the word “priority” in the dictionary?

No less deplorable is the briefing that Philip Hammond’s spring statement will dangle a bung of up to £20bn in additional public spending in front of MPs as an incentive to back his boss’s deal. This message has to be made public, of course, because the chancellor will be delivering his speech the day after the vote on the withdrawal agreement. The Commons needs to be informed of the incentive now, rather than on Wednesday. We have already seen this minority government disburse £1bn of public funds to buy the votes of the Democratic Unionist party. Is it seriously proposing to spend 20 times that amount to force through its disastrous Brexit deal?

The ERG leader Jacob Rees-Mogg at a rally organised by Leave Means Leave. Photograph: Daniel Leal-Olivas/AFP/Getty Images

Labour, meanwhile, shifts and shuffles on the question of a public vote, often appearing to care less about the consequences of Brexit for the country than the impact of its ever-morphing policy in leave-voting seats. After Jeremy Corbyn’s version of the deal was rejected by the Commons on 27 February, the ground was apparently set fair for the party to embrace, at last, a fresh referendum – as per the sequence agreed at its conference last year.

Yet on the BBC’s Andrew Marr Show on Sunday, John McDonnell seemed to imply that he and his colleagues would still be pressing parliament to endorse their own alternative to May’s agreement. So which is it? Will the real version of Labour’s referendum policy please stand up?

Precisely when a clear sense of the public interest matters most, both main parties are locked in deep introspection – staring at the ground, in fear of the sun. For now, the identity of the next Conservative leader and “the difficult balance that Jeremy has to strike between remain and leave voters” should be second- or third-order issues. This week’s parliamentary decisions will determine what sort of country we live in for decades to come. In my book, that counts for more than the sensitivities of the European Research Group and tribal Corbynites.

Assuming that there is indeed a vote on Tuesday – and experience teaches us that No 10’s assurances cannot always be trusted – it is inconceivable that May will present the once-rejected deal unamended. There will, at minimum, be a new Post-it note, scribbled on by Michel Barnier, promising that the EU will be nice if it has to deploy the Irish border “backstop” mechanism.

But it is equally inconceivable that these reassurances, whatever they are, will satisfy the hardline Brexiteers. If it is put to the vote, the deal will fail (again).

On Wednesday – according to the present timetable – the Commons will vote on “no deal”, and will almost certainly mandate the government to rule it out. This, in turn, will force May to do what she has most dreaded, which is to give parliament the chance to seek an extension of article 50.

And it is at this point that the character of our political class should truly be judged. Will it seek a meaningless three-month deferral, as the PM hopes, or – in dialogue with the EU – ask for a longer period, in which parliament can pause, draw breath and, if it sees sense, give the voters a chance to break the impasse?

There are no risk-free options now, no steady-as-she- goes, old-fashioned British compromise. The whole issue has to be reframed, re-energised and rescued from its present captivity. Jeremy Hunt, the foreign secretary, told Marr that the deal’s failure this week would risk “Brexit paralysis”. But where do he and his colleagues imagine we are right now?

• Matthew d’Ancona is a Guardian columnist