The previous year of elections had seemed to demonstrate that championing Brexit and promising to clamp down on terrorism by curtailing human rights laws would lead her Conservative Party to victory.

That voters did not simply hand her a defeat, but shifted to the center-left Labour Party when it is led by a left-wing populist, Jeremy Corbyn, once again confounds today’s understanding of the populist moment.

She was hardly the first politician to succumb to a false belief that she understood populism. David Cameron, her predecessor, held last June’s referendum on leaving the European Union believing it would co-opt nativist sentiment that was seen as too marginal to prevail.

In the United States, Republican Party leaders, and by some accounts even Donald J. Trump’s own campaign, all underestimated the anti-establishment swell that pushed him to the presidency in the 2016 election.

And in the Netherlands this year, the center-right prime minister, Mark Rutte, sought to contain populism by co-opting some of its harsh attitudes toward immigrants, but went on to lose one-fifth of his party’s seats in Parliament.

Some surprises have bolstered establishments, underscoring that the only certainty is uncertainty.

In Dutch elections, the far-right leader Geert Wilders fell significantly short of expectations, suggesting that even populists are struggling to harness populism.

In France, after the far-right candidate, Marine Le Pen, won more votes in the presidential race than her party ever had before, many expected the party would gain in coming legislative elections. But polls suggest it is heading for catastrophic defeat.