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.