LONDON — Britain and the United States have often seemed lashed together amid the populist storms of the last few years — Brexit and the Trump White House echoing and amplifying each other across the Atlantic. But in one respect they have radically diverged.
In London, rebels in the Conservative Party staged a dramatic insurrection in the past week against Prime Minister Boris Johnson, blocking his plan to withdraw Britain from the European Union even without a deal. In Washington, scarcely a handful of Republicans have stood up to President Trump, even when he has flouted party orthodoxy on issues like trade, immigration and the deficit.
The Tory party’s revolt against Mr. Johnson, and his ruthless purging of the rebels, are reverberating through British politics, threatening his hold on power. For dispirited Republicans, though, this British revolution has become an object lesson in how a center-right party can stand up to a wayward leader.
The Conservative rebels “showed courage and principled concern about the impact of bad policy on the U.K. economy,” said Daniel M. Price, who served as an economic adviser to President George W. Bush. “This contrasts with congressional Republicans here who have mostly been meek, mute or complicit.”
The uprising in Westminster came even though British political parties enforce discipline far more strictly than their American counterparts. Mr. Johnson punished the 21 renegades by throwing them out of the party. Mr. Trump can ostracize Republican dissidents and dry up their funding, but he cannot expunge them from the party rolls.
Much of the difference, experts said, has to do with a magnitude of the crisis on each side of the Atlantic. The Tories who broke with Mr. Johnson regard his vow to take Britain out of the European Union on Oct. 31, come what may, as so reckless that it poses a dire threat to the nation — one that would wreak economic havoc and sunder both their party and British society.
Analysts warn that a no-deal Brexit, one in which Britain abruptly leaves Europe without transitional arrangements on trade or borders, will lead to shortages of food and medicine, trucks backed up on both sides of the English Channel, and the threat of violence in Northern Ireland, where a hard border could reignite the sectarian troubles that all sides thought they had left behind.
“To deliver Brexit like this is to create a poison pill which for 40 years will divide this country straight down the middle,” Rory Stewart, a rising star of the Conservative Party, said in a BBC Radio interview.
Mr. Stewart, who challenged Mr. Johnson for the Tory party leadership in June, learned of his expulsion from the party via a text message on Tuesday just minutes before GQ magazine honored him as its politician of the year.
While many Republicans deplore Mr. Trump’s divisive language and erratic conduct, few accept the argument — at least publicly — that he poses a comparable threat to the United States. However distasteful they find him, Republicans largely back his agenda, whether it is the appointment of conservative judges, the passage of tax cuts, or deregulation.
They are even willing to tolerate his overturning of traditional Republican priorities like free trade, in part because of the damage they fear a vengeful Mr. Trump could do to them personally at the polls. The president has thoroughly taken over the Republicans, remaking the party of Lincoln in his image and institutionalizing policies that, only a few years ago, would have seemed extreme to them.
Mr. Johnson wants to engineer a similar takeover of the Conservatives, purifying the party of Winston Churchill and Margaret Thatcher so that it can repel challenges from the hard-core pro-Brexit movement, which now has its own competing party. If he clings to power, a more radicalized Tory party could yet emerge.
But Mr. Johnson offers little to supporters beyond a promise to leave the European Union next month. His other policies — tax cuts, more money for the police, tighter immigration rules — are standard-issue Conservative fare. Several of his rivals for the party leadership this spring ran on substantively similar platforms.
“There’s not much of a quid pro quo there,” said Tim Bale, a professor of politics at Queen Mary University of London.
In fact, Mr. Bale noted, a few of the rebels — notably Philip Hammond, who advocated a policy of austerity as chancellor of the Exchequer in the previous Conservative government — were put off by Mr. Johnson’s profligate spending plans.
While Mr. Johnson’s flamboyant image and populist appeals bear a surface similarity to Mr. Trump, he has not mobilized a grass-roots political movement anywhere near that of the president. Nor does he enjoy the prerogatives of a presidential system with a fixed four-year term. This past week, he wasn’t even able to call an election without the assent of the opposition Labor Party.
“Trump got elected; he got through the fires,” said Alan K. Simpson, a former three-term Republican senator from Wyoming who served as his party’s whip. “Unkempt Boris just showed up on the scene. You can rough up a guy like that.”
Mr. Johnson’s first foray into Parliament as prime minister was an unmitigated disaster. He lost four key votes in a row and faces the specter of having to do something he vowed he would never do: ask Brussels for an extension of the date when Britain will leave the European Union.
Until last week, Mr. Johnson, as a new prime minister, would at least have had the reliable backing of his party.
“There is a very strong sense of party loyalty,” Mr. Bale said. “Most M.P.’s recognize that they owe their seats to the party, not to themselves. It is a measure of the depth of feeling that so many of them stood up.”
The only precedent for this kind of rebellion came in the 1990s when a group of Conservative renegades opposed Prime Minister John Major’s attempts to implement the Maastricht Treaty, which tightened the bonds of the European Union. It was essentially the reverse of the current civil war: a cabal of Euroskeptic lawmakers defying a moderately pro-European government.
Mr. Major survived the challenge, but his party was weakened by the infighting and lost a general election in 1997. That is a worrisome precedent for today’s Conservatives given the stature of those who rebelled against Mr. Johnson.
In addition to Mr. Hammond and Mr. Stewart, the group included Kenneth Clarke, the most senior member of the House of Commons; David Gauke, a former attorney general; and Nicholas Soames, the grandson of Churchill. Even Mr. Johnson’s brother, Jo Johnson, resigned, saying he was “torn between family loyalty and the national interest.” Amber Rudd, the work and pensions secretary, on Saturday became the latest to quit.
A comparable uprising on Capitol Hill would require Senators Chuck Grassley of Iowa, the most senior Republican; Lamar Alexander of Tennessee, who served in Mr. Bush’s cabinet; Tom Cotton of Arkansas, a rising star on the right; and Mitt Romney of Utah, whose father ran for the Republican presidential nomination and who himself was the party’s candidate in 2012.
It is not that Republicans have never turned on their own president. In August 1974, at the climax of the Watergate scandal, three leading Republicans, led by Senator Barry Goldwater of Arizona, told President Richard M. Nixon that his support in Congress had evaporated. The next evening, Nixon announced his resignation.
American Republicans vote against their party far more regularly than British Conservatives. Senator Jim Jeffords of Vermont made a career of voting against fellow Republicans on issues like tax cuts, arms control and the impeachment of President Bill Clinton before leaving the party in 2001 and starting to caucus with the Democrats.
Some Republicans, like Senator Pat Toomey of Pennsylvania, have come out against Mr. Trump’s trade war with China. Others, like former Senator Bob Corker of Tennessee, have raised alarms about his handling of national security.
Still, after watching Republicans stand by Mr. Trump in the Russia investigation and the furor over his remarks after the racial clashes in Charlottesville, Va., analysts have largely given up any expectations of wholesale defections.
To this day, only a single Republican lawmaker, Representative Justin Amash of Michigan, has called for Mr. Trump to be impeached (he later left the party and is now an independent). Outspoken critics, like Mr. Corker and Senator Jeff Flake of Arizona, are now in retirement. Senator John McCain of Arizona, perhaps Mr. Trump’s biggest scourge in the party, died last year.
“If Trump proposed to do something really radical, like pulling out of NATO, maybe you would see some Republicans stand up,” said Thomas Wright, the director of the Center on the United States and Europe at the Brookings Institution. “But Republicans are largely happy with the legislative items being sent to the Hill.”
The Conservative Party is in a far more precarious position. It faces a threat from the insurgent Brexit Party, which could peel away many of its hard-core pro-Brexit voters if the party is perceived as weak on leaving the European Union. That helps explain why Mr. Johnson is so determined to rid the party of its dissidents.
“For us, the big picture is the future of the U.K. and Britain’s place in the world,” Mr. Wright said. “For the Tories, everything is about domestic politics. That’s why you’re seeing almost French Revolution-style radicalization of the movement.”