Like most enduring parties, the Tories have embraced many different factions and ideologies over the years. But in the post-World War II era, they were defined by an advocacy of free markets and traditional values — a combination that was brought to its climax in the person of Margaret Thatcher, the Tories’ most effective prime minister since Winston Churchill.

The free-market orientation made sense. The second half of the 20th century was dominated by one big issue — the clash between communism and capitalism. Throughout the world, parties aligned themselves on a left-right spectrum that related to that central issue: the role of the state in economics. In the United States in the 1950s and 1960s, the Democrats included Northern progressives and Southern segregationists, but they generally agreed on the need for an interventionist state.

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We are living now in a new ideological era, one defined by an “open-closed” divide — between people comfortable in a world of greater openness in trade, technology and migration and those who want more barriers, protections and restraints. Parties of the future will likely be positioned along this new spectrum.

You can see the breakdown of the old order by looking back at Britain’s previous five prime ministers, two from the Labour Party and three from the Tories. All were in favor of Britain staying in the European Union. (Theresa May had voted to remain in the E.U. , but once the “leave” side won the referendum, she promised to carry out the will of the people and take her country out of the union.) By contrast, Prime Minister Boris Johnson is remaking the Tories into the party of Brexit and this week expelled 21 members of Parliament from the Conservative Party, including very senior figures, who disagreed with the new party line.

Many commentators in Britain have pointed to the analogies between now and 1846, when Prime Minister Robert Peel pushed through a free-trade agenda that split the Conservative Party and kept it mostly out of power for a generation. No analogy is perfect, but when a party divides over a big issue — as did, for example, the U.S. Whigs over slavery — it usually narrows its political base and electability. There hasn’t been a Whig president in the United States since Millard Fillmore left office in 1853.

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Of course, not every situation will fall neatly on the open-closed spectrum. Many of the leading Brexiteers are staunch free marketeers and insist they want a “global Britain.” It is odd, however, to be in favor of free trade and yet insist that Britain crash out of the E.U., one of the world’s largest free-trade areas — and the United Kingdom’s largest trading partner.

More significant is the fact that whatever the views of the new Tory leaders, the people who voted for Brexit — and who would presumably support what would essentially be a new Tory-Brexit Party — largely embrace a closed ideology. They are suspicious of foreigners and resentful of the new, cosmopolitan Britain that they see in London and the country’s other big cities. They want less immigration and multiculturalism. They are more rural, more traditional, older and whiter and want some kind of a return to the Britain in which they grew up.

The United States, of course, has a similar constituency. While many of the Republicans who support President Trump might well be free marketeers, his base is largely animated by the same suspicions and passions that motivated the Brexit voters. Trump himself is an ideological omnivore — supporting free markets while simultaneously imposing the biggest tariff hikes since the Smoot-Hawley Tariff Act of 1930. The most likely future for the Republican Party is one that conforms with its voters’ preferences — for limits on trade and immigration and greater hostility toward big technology companies.

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In Britain, there is confusion on the other side of the aisle as well. The Labour Party has moved leftward and still contains elements that are skeptical about the European Union. Over time, Labour will probably move more robustly in a pro-Europe direction and, with the Liberal Democrats, try to create a new “open” governing majority. In the United States, the Democrats have to resolve similar differences mostly around trade, an issue on which many Democrats are as protectionist as Trump.

But what is happening now in Britain is a telltale sign. One of the world’s most enduring political parties is cracking — yet another reminder that we are living in an age of political revolutions.