Sometimes U.S. and U.K. Politics Seem in Lock Step. Not This Year.

A Conservative British prime minister sets the date for a long-awaited vote in the early summer and the United States follows with a momentous presidential election a few months later. It happened in 2016, when Britons voted for Brexit and Americans elected Donald J. Trump, and now it’s happening again.

Political soothsayers might be tempted to study the results of Britain’s July 4 general election for clues about how the United States might vote on Nov. 5. In 2016, after all, the country’s shock vote to leave the European Union came to be seen as a canary in the coal mine for Mr. Trump’s surprise victory later that year.

Yet this time, past may not be prologue. British voters appear poised to elect the opposition Labour Party, possibly by a landslide margin, over the beleaguered Conservatives, while in the United States, a Democratic president, Joseph R. Biden Jr., is in a dogfight with Mr. Trump and his Republican Party.

“We’re just in a very different place politically than the U.S. right now,” said Robert Ford, a professor of political science at the University of Manchester. The Conservatives have been in power for 14 years, Brexit has faded as a political issue, and there is no British equivalent of Mr. Trump.

To the extent that there is a common theme on both sides of the Atlantic, said Ben Ansell, a professor of comparative democratic institutions at Oxford University, “it’s really bad to be an incumbent.”

By all accounts, Mr. Sunak decided to call an election a few months early because he does not expect Britain’s economic news to get any better between now and the fall. Trailing Labour by more than 20 percentage points in polls, Mr. Sunak, analysts said, is betting that the Tories can cut their losses by facing the voters now.

Though there is little evidence that the American political calendar played into Mr. Sunak’s decision, holding an election on July 4 has the ancillary benefit of avoiding any overlap. If he had waited until mid-November, as political oddsmakers had predicted, he would have risked being swept up in the aftermath of the American results.

Political analysts were already debating whether a victory by Mr. Trump would benefit the Conservatives or Labour. Some postulated that Mr. Sunak could seize on the disruption of another Trump presidency as a reason to stick with the Tories, if only because they might get along better with Mr. Trump than Labour’s leader, Keir Starmer.

Now that is irrelevant: Britain will have a new Parliament, and very likely a new prime minister, before the Republicans and Democrats even hold their conventions.

Still, Britain’s election results could hold lessons for the United States, analysts said. The countries remain politically synchronized on many issues, whether it is anxiety about immigration, anger about inflation or clashes over social and cultural issues.

“Imagine there is a collapse of the Conservatives, like in Canada in 1993,” said Professor Ansell, referring to a federal election in which the incumbent Progressive Conservative Party was all but wiped out by the Liberals and even elbowed aside by the Reform Party as Canada’s major right-wing party.

Britain’s Conservatives face a milder version of that threat from Reform U.K., a party co-founded by the populist Nigel Farage, which is running on an anti-immigration message. In the latest poll by YouGov, a market research firm, Reform was at 14 percent, while the Conservatives were at 22 percent and Labour at 44 percent.

A surging Reform U.K., Professor Ansell said, “might be a sign that populism is back on the rise in the U.K., and could be an omen and portent that the same might happen in the fall in the U.S.”

Conversely, he said, major gains by Britain’s center-left parties — Labour, as well as the Liberal Democrats and the Greens — might reassure Democrats that their better-than-expected results in midterm and special elections were not a fluke but part of a larger global swing.

Some right-wing critics blame the Conservative Party’s decline on the fact that it has drifted from the economic nationalism that fueled the Brexit vote and the party’s victory in 2019 under then Prime Minister Boris Johnson. The Tories’ embrace of liberal free-market policies has, they said, put them out of step with Mr. Trump’s MAGA legions, as well as right-wing movements in Italy and the Netherlands.

“Whatever you think about Trump — he’s unstable, he’s a danger to democracy — if you look at how he’s polling, he’s doing a hell of a lot better than the Tories are,” said Matthew Goodwin, a professor of politics at the University of Kent.

Part of the difference, of course, is that Mr. Trump has been out of office for nearly four years, which means that he, unlike the Tories, is not being blamed for the cost-of-living crisis. Nor is he being faulted for failing to control the border, as Mr. Biden is in the United States and Mr. Sunak is in Britain.

In his bid to mobilize the Conservative base, Mr. Sunak is sounding notes that echo the anti-immigrant themes of Brexit campaigners in 2016. He has spent much of his premiership promoting a plan to put asylum seekers on one-way flights to Rwanda. Costly, much criticized, and unrealized, it has more than a little in common with Mr. Trump’s border wall.

“This has been kind of our Trump moment,” said Kim Darroch, a former British ambassador to Washington. “But given the legacy that Keir Starmer will inherit, you can’t rule out someone from the right wing of the Tory Party exploiting a weak Labour government to get back into power in four or five years.”

For all its totemic importance, Brexit has scarcely figured as an issue in 2024. Analysts said that reflects voter exhaustion, a recognition among Tories that leaving the European Union harmed Britain’s economy, and an acceptance the Britain is not rejoining anytime soon.

“You’re not allowed to talk about Brexit because both parties are terrified about what happens if you take the dog off the leash,” said Chris Patten, a former governor of Hong Kong and Conservative politician who chaired the party in 1992, when it overcame a polling deficit to eke out a surprise victory over Labour.

Mr. Patten said he was skeptical that the Conservatives would pull that off this time, given the depth of voter fatigue with the party and the differences between Mr. Sunak and John Major, the prime minister in 1992.

Tory members of Parliament seem to share that sense of futility: Nearly 80 of them have opted not to contest their seats, an exodus that includes Michael Gove, who once vied for party leader and has been at the heart of nearly every Conservative-led government since David Cameron’s in 2010.

Frank Luntz, an American political strategist who has lived and worked in Britain, said the elections in Britain and the United States were being driven less by ideological battles than by a widespread frustration with the status quo.

“We’re in a completely different world than in 2016,” Mr. Luntz said. “But the one thing that both sides of the Atlantic have in common is a feeling that can be summed up in one word: enough.”