Government forecasts suggest a hard Brexit may trigger a deep recession, and some say it will wreck the party. As one Conservative minister said in April, if the party advocates for a no-deal Brexit, “we’re saying goodbye to young people, goodbye to Remain voters, and goodbye to the center ground of British politics.”

Mr. Johnson has vowed that the U.K. will leave the EU on Oct. 31 “do or die,” and embraced a no-deal Brexit if necessary. Polling suggests that Conservative members, who are older, whiter, and less urban than the broad electorate, support a “hard Brexit.” In May, the Brexit Party polled first in largely symbolic European Parliament elections; the Conservatives finished fifth.

In the United Kingdom, tension between the Tory base and the elite has echoes of the tea party revolt a decade ago that reshaped the GOP and blazed a path for Donald Trump’s presidency. Here, as in the United States, the revolt is as much about culture and identity as it is about economic policy. And in Mr. Johnson, an Eton-educated populist who became the face of Brexit in the 2016 referendum, it may have found its man – a bridge between a Brexit-at-all-costs base and a flailing party establishment.

A quarter-century of agitating at the grassroots of Britain’s oldest and most successful political party has taught John Strafford a few tricks.

Meet regularly but not too often. Budget modestly: Rent a cheap space and serve a simple lunch. And always pack a spare kettle for endless cups of tea, just in case.

On a recent Saturday morning, Mr. Strafford, a retired accountant and entrepreneur, set out four rows of chairs in a small church hall in this leafy London suburb of million-dollar houses and stalwart votes for the Conservative Party, of which Mr. Strafford is a lifelong member. His small group of party activists was gathering to discuss, among other matters, a national leadership contest: It is widely assumed the winner will be Boris Johnson, who is set to replace Prime Minister Theresa May on Wednesday.

But Mr. Strafford and his allies are not content with merely voting for their next leader. They want a much greater say going forward in how the party is run and who stands for office, particularly in safe seats in the heartland for the Tories, a center-right party which has its roots in a 17th-century parliamentary faction and styles itself as the natural party of government. And when it comes to the defining issue of Brexit, that means purging Conservative members of Parliament who stand in the way of a hard break from Europe.

Mr. Strafford describes a long war of attrition between the party establishment and its corporate donors, and activists in towns and suburbs like this one. “Brexit brought it to a head. It was going to implode at some point,” he says.

That tension between the Tory base and the elite has echoes of the tea party revolt a decade ago that reshaped the GOP and blazed a path for Donald Trump’s presidency. Here, as in the U.S., the revolt is as much about culture and identity as it is about economic policy. And in Mr. Johnson, an Eton-educated populist who became the face of Brexit in the 2016 referendum, it may have found its man, a bridge between a Brexit-at-all-costs base and a flailing party establishment.

Crucially, Mr. Johnson is also seen as a vote-getter who can take on Labour, the party’s traditional left-wing foe, and the upstart Brexit Party that is snapping at its right flank. In May, the Brexit Party polled first in largely symbolic European Parliament elections; the Conservatives finished fifth.

For Jon Stanley, a fellow at the Bow Group, a right-wing think tank in London, the Conservatives have no choice but to embrace Brexit and stop trying to hold Remain voters.

“These two groups are now permanently aligned and if the Tories don’t choose the obvious group, which is the Leavers, it will be in trouble really, really fast,” he says.

“Do or die”

Around 160,000 Conservatives were eligible to vote in the monthlong run-off race between Mr. Johnson, a former foreign secretary and London mayor, and Jeremy Hunt, the current foreign secretary. Ms. May agreed to step down in May after failing to take the United Kingdom out of the European Union in March and being forced to negotiate a Brexit extension until Oct. 31.

Stefan Rousseau/PA/AP Back row from left, Charles Walker, Bob Blackman, Dame Cheryl Gillan, Nigel Evans and Geoffrey Clifton-Brown announce the results of the fifth ballot in the Tory leadership ballot at the Houses of Parliament in Westminster, London, Thursday, June 20, 2019. Boris Johnson and Jeremy Hunt will compete to become Britain's next prime minister in a runoff vote by members of the governing Conservative Party.

Mr. Johnson has vowed that the U.K. will leave the EU on that date “do or die,” and embraced a no-deal Brexit in the event that the EU refuses, as it has done until now, to amend his predecessor’s contentious withdrawal agreement. Ms. May’s own government forecasts that a “hard Brexit” is likely to trigger a deep recession; many U.K. businesses have repeatedly urged Conservative leaders to abandon the threat.

But polling suggests that Conservative members, who are older, whiter, and less urban than the broad electorate, support a hard Brexit. Mr. Strafford says any economic setbacks, even a drop in living standards, would be justified by the freedom and liberty that a clean break from Europe affords. “This is a gut feeling among the British public,” he says.

This feeling is not shared, however, by Dominic Grieve, a former attorney general who since 1997 has represented the Beaconsfield constituency – in which Gerrards Cross lies – and has threatened to bring down any Conservative government that pursues a no-deal exit. Which is why Mr. Strafford and other disgruntled local members are trying to stop Mr. Grieve standing again as their candidate. He is one of several pro-Europe Tory members of Parliament, including current and former ministers, facing deselection campaigns, though few have been as outspoken on Brexit as Mr. Grieve.

At the church hall meeting, which is attended by 18 mostly gray-haired men and women, Mr. Strafford provides an update on their campaign and how they can keep up the pressure. “It’s critically important that we win this battle so that the voice of members is heard,” he says.

U.K. parties don’t hold primaries and the process of deselecting a sitting MP is complex. In the case of Mr. Grieve, who lost a no-confidence vote in March, the Beaconsfield Conservative association has asked him to reapply to stand in the next general election. Still, warns Mr. Strafford, that mechanism probably won’t work in time for a snap election.

Asked by a Monitor correspondent for a show of hands as to who would vote for Mr. Grieve as their candidate in such a scenario, none go up.

Older and whiter

In seats like Beaconsfield, a drop in the Conservative vote may not sink the party. But the rise of the Brexit Party, perhaps the closest analog to the tea party, could prove disastrous for any Conservative leader trying to win a fresh mandate, particularly if Brexit is unfulfilled, as was shown in May when droves of Conservatives defected in the European parliamentary election.

“If we don’t come out they’ll stay with the Brexit Party, and then the Tory party becomes a rump,” warns Mr. Strafford.

Allies of Mr. Johnson have suggested that he could strike a one-off electoral pact with the Brexit Party in order to avoid splitting the anti-EU vote, as happened in a by-election in Peterborough in June that Labour held, despite only polling one-third of ballots cast.

Such a strategy is risky, though, since it would jettison moderate Conservative votes, say analysts. And it doubles down on the ideology of a base that is out of step with modern Britain in its preference for a no-compromise Brexit even if it wrecks the party and its brand.

Rory Stewart, a Conservative minister who ran as a centrist in the leadership race and failed to make the run-off, said in April that if the party advocates for a no-deal Brexit, “we’re saying goodbye to young people, goodbye to Remain voters, and goodbye to the center ground of British politics.”

Ahead of the leadership race the party’s membership has grown, but at 160,000 is a fraction of its postwar peak. As recently as 1970, it represented over 3% of the electorate and Conservative social clubs and events were a mainstay of town life. Now it is less than 0.4% of Britons.

The raw numbers may be less important than demographics; Conservative activists skew older and whiter, while their fervor for Brexit is failing to win over the next generation of voters.

“They’re increasingly locked out of urban Britain, which is younger and more multicultural,” says Tim Bale, a professor of politics at Queen Mary, University of London who has surveyed the party’s membership.

He points out that an aging Tory base in prosperous towns finds it easier to dismiss as fearmongering the forecasts of a no-deal Brexit recession. “They’ve got their house. They’ve got their pension. The state of the economy isn’t so much of a worry for them,” he says.

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Maureen Holding, a Conservative councilor in the New Forest in southern England, says Mr. Johnson is the right leader for the party as he can speak to younger voters. Just as important, says Ms. Holding, who defended her seat in a May council election that saw a surge in votes for the pro-EU Liberal Democrats, he’s committed to leaving the EU on Oct. 31.

Asked about the economic disruption of such an act, Ms. Holding raises her chin. “We are Great Britain,” she says.