When Dominic Grieve, a Conservative MP and former attorney-general, was heckled by party members shouting “traitor”, the forces ripping through his party were plain for all to see.
Mr Grieve, who lost a vote of confidence brought by his local party association on Friday, is one of a clutch of pro-EU Tory MPs targeted for deselection by local members.
He has blamed his reversal on “entryism” by anti-EU activists into the local party, but the transformation in the Conservatives’ ranks goes far beyond his Beaconsfield constituency.
By any measure, the party is now more Eurosceptic than at any time in its history, presenting Theresa May, prime minister, with an agonising dilemma as she seeks to salvage her unloved Brexit deal.
“The Tories have always had a tension between the interests of big business against the importance of national sovereignty,” said Paul Goodman, editor of the ConservativeHome website. “What has happened this time is that over the last 25 to 30 years national sovereignty has won out.”
In the 1990s John Major, the then Tory prime minister, saw his premiership suffocated by a hard core of only about 20 anti-EU MPs, whom he memorably described as “bastards”.
This weekend, by contrast, some 170 MPs — including 10 cabinet ministers — signed a letter urging Mrs May to back a no-deal Brexit if necessary by a deadline of May 22.
It is a stance many Conservative activists support. One poll in January by Sussex University and Queen Mary University found the party’s grassroots preferred no deal to Mrs May’s agreement by a margin of 64 to 29 per cent.
Mrs May now faces the huge challenge of trying to get her deal through the House of Commons — without provoking a revolt in what has largely become a Brexit party.
After three Commons defeats for her exit agreement the prime minister could in theory try to find a majority by accepting long-term plans for a softer Brexit. Perhaps the leading option is a customs union, one of several rival plans on which MPs are set to vote on Monday.
But, because of the Tory shift, any move by Mrs May to soften her Brexit plans could lead to an outright schism within the party.
“Parliament is going to keep rejecting the deal, we’re going to end up with a permanent customs union and split in the Tory party,” predicted one senior cabinet minister.
Over the weekend, many leading Tories sought to overcome the internal strains in the party, as senior party figures including Boris Johnson, a prominent Eurosceptic, rallied behind Mr Grieve.
But Mr Goodman of ConservativeHome argues that the tensions are part of a long-term trend in which every 70 years or so the Tories endure a “colossal political, economic and constitutional bust-up” over protectionism versus free trade.
In 1846 the Tories under Robert Peel split over the repeal of the Corn Laws — tariffs that protected landed interests by pushing up food prices. In the early 20th century the party was convulsed over the issue of “Imperial Preference”, favourable trade terms with the rest of the British empire.
Today many Conservative members share a distaste for globalisation and unease about the increase in UK immigration that followed EU enlargement.
Chris Patten, a former Conservative cabinet minister and EU commissioner, argues that the party — which has many fewer members than the Labour opposition — has become more extreme as its base has shrunk in size.
“We have these individuals who are economically illiterate jihadis, Maoists, groups who are focused on an idea of sovereignty which is mythical,” he said. “It is a variation of English nationalism that undermines the old Conservative belief in the United Kingdom.”
While the party downplays suggestions of infiltration, some MPs say that many former members of the anti-EU UK Independence party are joining their associations. “My local party is gearing up for deselection and already parading my replacement,” said one.
Alistair Burt, a Europhile who recently stepped down as a Foreign Office minister, warned on Sunday about the damage if associations became “polarised”.
“Political parties are fragile . . . we should fight hard to keep the broad nature of our party,” he said.
But many Conservatives say that a permanent customs union with the EU is in any case unacceptable, because it would curtail Britain’s ability to negotiate trade agreements — one of the chief motivations for Brexit. Only 33 Tory MPs backed the idea in parliamentary votes last week.
Grant Shapps, a former chairman of the Conservative party who supported Remain in the 2016 EU referendum, said that it “simply beggars belief that any MP could vote for a customs union”, calling it an “insane outcome”.
“I just can’t support a customs union, if that’s where we end up there will be carnage,” said one MP who backed Mrs May’s deal last week. “The party would be very angry. Some may break away.” Another said that backing a customs union would be “explosive”.
A third Eurosceptic told the FT that some ministers were “playing with fire” by trying to swerve towards a softer Brexit. “Of course, the rightwing will be blamed as usual but . . . it will be helpful to highlight the true arsonists.”
Some Tories doubt whether Mrs May is decisive or strong enough to make the historic choice between building a coalition for a deal at the cost of a softer Brexit, or keeping her party intact. And, with the UK’s exit from the EU now scheduled for April 12, time is running out.
One senior figure from the European Research Group, the most prominent group of pro-Brexit Tory MPs, said “the angry British people” wanted a general election. Many other MPs think that would be electoral suicide.
“It’s a madman’s game to guess what will happen,” said one ministerial aide.
Lord Patten believes the party is heading for existential “mayhem” at the hands of Eurosceptics. “I have never believed that it was possible to act in the national interest on Brexit and the EU and keep the Conservative party together,” he said. “I don’t think it can be done.”