The EU referendum and its aftermath have seen a role reversal in the Commons.  Former Ministers who backed Remain, lifelong loyalists to the Party orthodoxies, have gradually learned to operate as dissidents and occasional rebels: Stephen Hammond, Dominic Grieve, Nicky Morgan, above all Ken Clarke.  Former backbenchers who supported Leave, some of whom have never held Ministerial office, are suddenly the new Party establishment, fully supportive of the Government’s EU negotiation aims as first set out by Theresa May: “So it is not going to a “Norway model”. It’s not going to be a “Switzerland model”. It is going to be an agreement between an independent, sovereign United Kingdom and the European Union…We are not leaving the European Union only to give up control of immigration again. And we are not leaving only to return to the jurisdiction of the European Court of Justice.”

Senior Brexiteers and other longtime Leave backers have expressed concern from time to time.  In January, in the wake of the Prime Ministers’s interim deal with the EU, Theresa Villiers warned that “the direction of travel seems to have gone in only one single direction: towards a dilution of Brexit”. In March, Jacob Rees-Mogg said, during the aftermath of the agreement about transition, that “this agreement gives away almost everything and it is very hard to see what the Government has got in return.

But, by and large, the committed Eurosceptics in the Commons have done what establishments do: namely, to operate behind the scenes – lobbying Ministers, warning Whips, using the Party’s internal mechanisms to make their views known.  Downing Street is keen to keep it that way.  So it is that pro-Brexit partisans are being asked by the whips to keep shtum, or at least muted, during next Thursday’s Commons debate on the Customs Union.  Let pro-Customs Union Tories huff and puff, the Brexiteers are being told.  Let them talk themselves out.  Nothing will change , as the Prime Minister would put it.  Post-Brexit, the UK wil leave the Common Commercial Policy and Common Commercial Tariff, and gain the right to negotiate and sign trade deals.

Those strong Eurosceptics are not so sure.  The Government’s Customs Union defeat in the Lords may have been expected in Westminster, but it has reminded Leave supporters of what they are up against.  There is no continuity Vote Leave campaign.  By contrast, there are lots of noisy former and present Remainers.  Their impact is greater in the Westmister Village than outside it – but that village is where most MPs spend a lot of their time.

What might be called the Official Wing of the former Remainers recognises that Brexit is set to happen.  So it is now concentrated on what it believes will limit the damage.  Originally, its aim was to stay in the Single Market.  The idea won’t fly because Labour won’t back it: too many of the party’s northern and midlands MPs are worried, quite rightly, that continued Single Market membership would be incompatible with a proper system of immigration control.  So the Official Wing has fallen back on continued Customs Union membership, or something so like it to make no difference.  Hence the Government’s defeat in the Lords this week.  Hence Thursday’s debate – and pro-Customs Union amendments when the Trade Bill comes to the Commons.  The aim is to wear the Government down.

The EU knows this – hence the timing of its rejection of May’s Customs Union ideas.  It is piling on the pressure – particularly over the UK-Ireland border by rejecting the Government’s ideas for resolving most of the big potential problems through waivers and technology.  It hopes that influential officials, such as Olly Robbins, May’s Brexit adviser, will be less committed to leaving the Customs Union than many Tory politicians.  Even some pro-Leave Ministers are saying sadly that there is no Commons majority for leaving that union.

This site is sceptical of whether Brexiteers would really reject any agreement that the Government eventually reaches,  however unsatisfactory they may find it.  Conservative MPs are unlikely to take collective action that might risk the emergence of a far-left Labour Government – whatever the Fixed Terms Parliament Act may say.  And the committed Leavers are unwilling to take the blame for ousting a leader in whom committed Remainers also have no confidence as a future election winner.  But if Downing Street is counting on detestation of Corbyn, plus a lack of support in the Commons for No Deal, to stay the formers’ hand, it should think again.  For some of those Brexiteers, the collapse of May’s position would be a bridge too far.

“I’m not saying that there would be an organised push, but the letters would just go in to Graham Brady,” one senior pro-Leave backbencher told this site yesterday.  The sense is that in these circumstances some Brexiteer MPs have had enough – not necessarily as an organised collective, but certainly as angry individuals.  “If there’s a cave-in on the Customs Union, I think there will be aleadership challenge,” said another.  Perhaps it’s just talk: after all, there was talk of a challenge to David Cameron for most of the Coalition years.

But Brexit is a wild card.  And while staying in the Customs Union, or joining a customs union, might not be a breach of the letter of the referendum, it would certainly be one of its spirit.  Then there are the complicated cross-currents of the ambitions of future leadership aspirants.  We suspect that Number Ten is under no illusions about the fragility of the position.  There is a danger of mistaking silence for consent.