The European Research Group is attracting increasing amounts of attention with its crusade against the proposals Theresa May hammered out at Chequers for Brexit Britain's future relationship with the European Union.
Jacob Rees-Mogg, chairman of the group, and his Brexiteer colleagues have insisted so far that all they want is for Mrs May to "chuck Chequers". But the lengthy discussion 50 of them had on Tuesday night about "how best do we get rid of her" will not allay suspicions that they threaten to put an early end to her leadership.
For now, the ERG is focusing its fire on Mrs May's Brexit proposals, revealing today how they would prefer to answer the Irish border question. But how much damage could they cause her if push comes to shove? It will depend on the numbers they can muster in support, so it's worth reviewing how Tory Brexiteers divide in the party.
How big is the ERG?
The ERG is estimated to have as many as 80 Tory MPs involved. That, incidentally, is how many MPs Steve Baker – who quit as Brexit minister in protest over Chequers – suggests could march through the voting lobbies against it.
So far, just over 40 MPs have publicly declared their opposition to Chequers. Official records show that a few cabinet members such as Penny Mordaunt and other government ministers have been subscribed to the ERG, although collective responsibility restricts how actively they can resist Chequers right now.
What type of Brexit does the ERG want?
Tory members have united behind a Canada plus model. Failing that, they want the United Kingdom to embrace WTO rules as what they call a "World Trade Deal".
The appeal of a no-deal Brexit is one of the major points of disagreement. Backbenchers like Andrew Bridgen, Peter Bone and John Redwood are much more receptive to life under WTO rules. "Lots of the debate is driven by sound and passionate principles", a Eurosceptic source tells me.
Their tenor has drawn bemusement from former Conservative and Ukip MP Douglas Carswell, who has lamented on Twitter how the ERG is "full of people implacably opposed to some perfectly sensible proposals by May". Some members might be partial to Chequers, I'm told, but "they will be keeping very quiet".
Does the ERG speak for all Tory Brexiteers?
No. Several, like Michael Gove and Dominic Raab, feel happy enough with Chequers to be able to support it. Others, like Andrew Percy, are so fed up by the quarrels that they have set up a "Brexit Delivery Group" pledging to help deliver a "pragmatic" result.
But the ERG can claim to speak to the broadest swathe of Tory Brexiteer MPs, with Mr Percy's Brexit Delivery Group boasting at most 50 MPs – and not all of them Eurosceptics.
Mr Rees-Mogg's can also claim to be the most effective Brexiteer pressure group, having successfully forced the Government to accept several amendments toughening up its red lines in July.
Will the ERG sweep in Prime Minister Johnson or Rees-Mogg?
Not by itself. The ERG has the numbers to force a no-confidence vote in Mrs May. With the Telegraph understanding that around 35 letters have been submitted demanding one, only 13 more need write in for it to happen.
But senior figures in the group are adamant that they do not want the Prime Minister to be ousted. Iain Duncan Smith told BBC Radio 5 today that there was "no coup" against Mrs May and if he found one, he "would stamp on it".
Another Tory MP who attends ERG meetings told me they were "astounded by how hysterical" some of their colleagues have been, adding: "You know, some of them are a bit thick." However, they conceded that if Mrs May's Chequers proposals deteriorate, they would be driven to similar fury about her handling of Brexit.
The ERG's refusal to endorse a coup illustrates that members know they can force a no-confidence motion, but they do not have sufficient numbers to ensure it passes – which would require just over half of Tory MPs to vote against the Prime Minister. Without any clear plan and support from other corners of the Tory party, it would be hard for them to crown their preferred Brexiteer as her successor.
So does that mean Mrs May is safe?
No. Her wafer-thin majority, only existing due to the help of her DUP allies, means the ERG easily has the numbers to disrupt parliamentary legislation. Any eventual deal will struggle to pass without the support of its members.
The DUP has endorsed the proposals put forward by Mr Rees-Mogg and his ERG colleagues for the Irish border today. So Mrs May should risk their wrath at her peril, as doing so could risk her Unionist allies pulling their support.