Mr. Krastev sees the divisions over migration most sharply between the countries of Western Europe like Germany and those of the east, like Poland, Hungary and the Czech Republic, which view “the very cosmopolitan values on which the European Union is based as a threat.” The populist revolt is not simply against mainstream parties, he argues, but against meritocratic elites who have arguably lost touch with their roots.

In Austria, which likes to think of itself as the bridge between the western and eastern countries of Europe, Mr. Kurz faces a serious choice: try to isolate a popular populist party, as in Sweden, or bring it into government and try to tame it, as in Norway. But with nationalist, populist parties already a part of several European governments, and in power in Hungary and Poland, the European Union has little appetite for imposing sanctions or other penalties.

As successful as Mr. Kurz was in reviving his own party — even changing its colors from black to turquoise — he failed to stem the appeal of the Freedom Party, which won roughly the same percentage of votes as it did in its previous heyday in 1999, under Jörg Haider, when its roots in provincial nationalism and neo-Nazism were more obvious.

Back then, the conservative Wolfgang Schüssel broke a taboo by bringing the Freedom Party into government, and Europeans reacted with horror. Without waiting for the new government to do anything, European governments imposed diplomatic sanctions on Austria.