Next week’s British election is said to be that nation’s most important since World War II. That’s true, but for Americans, it will also serve as a window into our own 2020 contest.

British politics have long mirrored our own. Both nations have been dominated for decades by two parties, one center-right — the Conservatives and the Republicans — and one center-left — Labour and the Democrats. The center-right parties historically attracted the educated and relatively well-off, while the center-left ones attracted the poor, the working class and ethnic and racial minorities. There were important differences, to be sure, but Democrats and Republicans could each see their British counterparts as first cousins rather than distant relatives.

This parallelism meant that trends observed in one country often presaged changes in the other. In the 1950s, both countries were governed by cautious conservatives, who protected the welfare states built by their opposition. Both nations turned leftward at roughly the same time in the 1960s. Both were beset by stagflation during the 1970s, and turned toward free-market conservatives — Margaret Thatcher and Ronald Reagan, both once thought beyond the pale — in the 1980s. And in the 1990s, New Democrat President Bill Clinton and New Labour Prime Minister Tony Blair pushed their parties away from their working-class roots and toward a market-friendly “Third Way.”

Both countries today are dealing with the populist revolt driven by the effects of the 2008 financial collapse, globalization and mass migration. Britain’s decision to leave the European Union in a 2016 referendum shocked the nation’s elites just as Donald Trump’s upset victory shocked U.S. elites a few months later. Brexit has become a cause celebre among the American right, as Trump acolytes say the Brexit vote presaged Trump’s own victory. And Britain’s Remainers, as those opposed to Brexit are known, never fail to try to tie the unpopular Trump to pro-Brexit Conservative Prime Minister Boris Johnson.

These factors are having similar political effects in both countries. Affluent, pro-Remain Tories are leaving their party for either Labour or the small, anti-Brexit Liberal Democrats just like affluent suburbanites are leaving the GOP for the Democratic Party. Working-class voters in England and Wales who want to leave the E.U. are moving in the opposite direction, just as their blue-collar counterparts in the United States elected Trump. The election’s outcome will largely be determined by which movement is stronger.

The parallels go on. Labour has lurched left in recent years, and its leader, Jeremy Corbyn, is an unrepentant socialist who criticizes capitalism. Democrats are also swinging left, as the rise of Rep. Alexandria Ocasio-Cortez (D-N.Y.) and the strength of progressive favorites Sens. Bernie Sanders (I-Vt.) and Elizabeth Warren (D-Mass.) in the presidential primaries show. Polls show voters under 30 in both countries largely favor these left-wing populists and that affluent anti-populist former conservatives are scared by them. Fear of, and hope for, dramatic change will be important factors in both countries’ elections.

The populist leaders’ character flaws are also election issues. Trump’s are well-known in both countries, but Americans are unfamiliar with just how out of the ordinary Johnson’s past is. Like Trump, Johnson is known for multiple marriages and many affairs. He is known to be petulant, brash and ingratiating. Johnson’s foes say he doesn’t have the sober temperament needed for the job, while his allies say his energy, charisma and out-of-the-box demeanor are exactly what the country needs. Sound familiar?

Johnson’s Tories are also challenging free-market orthodoxy just as Trump’s populism is leading Republicans to rethink their economics. The Tory manifesto eschews cuts in personal or corporate income taxes in favor of cutting a payroll tax paid by all. It also unleashes a flurry of government spending on services and projects favored by formerly Labour-voting working-class voters. If Johnson wins with a stable majority, this visionary platform will likely become an inspiration for post-Trump Republican reformers.

Threats to national unity are also increasing in both lands. In the United States, the threat comes from fears that increasing polarization and the vast gap in cultural attitudes between party partisans will irrevocably damage our democracy. In Britain, the threat is more direct: The United Kingdom is actually four countries — England, Wales, Scotland and Northern Ireland — and Brexit is raising the chances that Scotland and Northern Ireland will choose to leave. Polls earlier this year showed that Brexit supporters cared more about leaving the E.U. than maintaining British Union, and the Scottish Nationalist Party’s manifesto forthrightly plunks for another referendum on Scottish independence.

I’ll be writing from London and Britain’s flyover country over the next week reporting on the election campaign and describing each of the trends in detail. Whatever happens will not only determine the future course of our closest ally but also give us a clear and perhaps unsettling look into our own future.

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