LONDON — President Trump’s push for a border wall hints at a problem that populist leaders are facing across the Western world.

After a year of setbacks, populist leaders and parties are trying to rejuvenate their fortunes by revitalizing the sense of crisis on which they thrive. But as with Mr. Trump’s demand for a border wall — which has brought a two-week government shutdown — this may say more about populism’s weakness than its strength.

Immigration and terrorism crises, which aided populism’s world-shaking rise in 2016, have waned. Populists have faced disappointing election results in Germany, the United States and even Poland, shattering the image of the movement’s inevitability and its claims to represent true popular will.

The West’s populist leaders and parties have grown defensive, retreating into ever-starker messages of us-versus-them. The approach excites their most dedicated followers. But it can be risky, forcing voters to pick sides at a moment when the populist right holds declining appeal.

Populism is hardly dying. It holds power in the United States, Italy and a few Eastern European countries, as well as meaningful parliamentary minorities in much of Western Europe, where populist parties now reliably win about one in six votes.

Still, without a crisis to justify populism’s hard-line policies, its message has been stripped down to its most core element: opposition to liberal ideals of pluralism, multiculturalism and international cooperation.

The result is a new phase in the populist era, one that will test populism’s appeal — and that of its ideological rival, postwar establishment liberalism — as never before.

A Rocky Year

This story, playing out across Western democracies, may be encapsulated best by the still-unfolding drama of Mr. Trump, the government shutdown and the border wall.

Two years after he won the presidency, threats supposedly posed by immigration and terrorism in the United States have not materialized.

Illegal immigration continued its 10-year decline. A spate of terrorist attacks inspired by the Islamic State occurred before Mr. Trump took office.

Americans lost enthusiasm for strict policies like Mr. Trump’s promised wall on the Mexican border, which polls poorly. Republicans suffered a devastating defeat in the midterm elections. Mr. Trump’s divisive message, rather than pulling more voters in, had pushed some away.

But populists, who thrive on conflict against an existential threat, cannot bend with popular will as easily as mainstream parties. Rather than giving up on his wall and compromising his vision, Mr. Trump has dug in.

Populists in Europe had a similarly rocky year.

In Britain, support for Brexit has slipped below 50 percent. Polls suggest a majority of voters want a second referendum. Brexit hard-liners in the governing Conservative Party tried and failed to eject Theresa May, the prime minister, over her support for a softer Brexit.

In Germany, the rise of Alternative for Germany, a far-right party, has stalled. It performed worse than expected in elections in the border state of Bavaria, where immigration is a major issue, and worse than it fared a year earlier.

When Bavaria’s center-right party tried to co-opt the populist message and challenge Chancellor Angela Merkel over immigration, it suffered election losses. Ms. Merkel survived, her approval rating spiked and she lined up a centrist successor.

Brexit hard-liners are pointing to the 470 people who illegally crossed the English Channel by boat last year, a drop in the bucket compared with the hundreds of thousands of arrivals to Europe in 2015 and 2016. Mr. Trump has claimed, with no hard evidence, mayhem at the Mexican border.

But dividing the world into us versus them works only if voters want to belong to “us” and oppose “them,” typically establishment elites and cultural outsiders.

In Europe, many more voters have had contact with migrants since 2016, which research suggests can lessen fear and resentment. In the United States, the buoyed economy has undercut fears of economic competition from immigrants.

The Curious Case of Sweden

One of the countries most often cited as a populist success last year may also underscore the movement’s challenges.

The Sweden Democrats, a populist party, won 17.5 percent of the vote, its highest share ever, in a national election in September. If populists could surge this high, this fast even in Sweden, a bastion of liberalism, surely it represented a global shift.

But polls tell a different story.

Support for the Sweden Democrats has not grown since the end of 2015, just as the refugee crisis began tapering off. And the party’s share of the vote last year was only slightly more than that of Dutch far-right populists in 2017, which had been considered a disappointing setback for the movement.

A review of populism’s global performance, conducted by Jordan Kyle and Limor Gultchin of the London-based Institute for Global Change, reached a similar finding.

Populists today hold 20 governments worldwide — the same number they held in 2010, have held most years since and have never surpassed. What we perceive as a new wave may in fact be populism shifting out of poorer countries, often in Latin America, where they have since suffered setbacks, to the West.

But there is another way to read cases like Sweden: not as the populist wave cresting, but as the liberal consensus breaking. Even if populists win power only occasionally, struggle in office and mostly consign themselves to an angry minority, that they play any role at all represents a seismic change.

Their rise, even if it never progresses much further, could still reshape Western politics in ways we are only beginning to understand.

A Deeper Appeal

Stripped of claims to be responding to an immigration crisis, the populist message is taking on a certain clarity — and proving an enduring, if narrow, appeal.

Ever since, they have used crises to hit on deeper fears of demographic and cultural change brought about by liberalism. And they have channeled opposition to an establishment consensus that called it taboo to question liberal ideals.

There has always been some discomfort with the aspect of liberalism that populists are most aggressively challenging: the notion that democracies must extend rights and protections to outsiders.

Western populists have surfaced that discomfort and given it voice. By winning at least enough votes to force the world to listen, they have opened up space for Western voters to reject liberal strictures explicitly.

Parties like the Sweden Democrats and Alternative for Germany may no longer have much hope of leveraging crises to win power outright. But there appears to be enough latent opposition to liberalism to keep populists well involved in politics.

Border crises, real or imagined, are ideal for this message. They highlight aspects of liberalism that people find most objectionable: promises to protect outsiders, demands that countries compromise sovereignty and the softening of fixed, racially defined national identities.

So perhaps when Brexit hard-liners or Bavarian populists play up border crises that may seem exaggerated they are speaking to a deeper concern among their small but dedicated base.

That may be enough to keep them in national legislatures and conversations. Even if populists in the West never advance beyond their heights in 2016, they will remain in a strong position to challenge liberalism’s postwar hold over Western democracies.

Postwar liberal democracy is simply too new of a system, scholars of democracy say, to know whether it can survive these challenges. We may look back at 2016 as a populist blip associated with one-off crises, or as the beginning of a process of chipping away at liberal democracy from within.

“For anyone who was hoping for a break in the hectic politics of the past years,” Mr. Mudde wrote in his assessment of populism’s prospects, “2019 won’t be it.”