One of the first institutions the party hijacked was public television. Law and Justice has turned it into Fox News on steroids, paid for by the taxpayers. It feeds viewers nonstop propaganda about the mounting threat to Poland’s sovereignty from the European Union, specifically in the form of Muslim refugees.

Those refugees present a threat to our way of life, the government and the press insist. They will assault our women, they say, and they are carrying infectious diseases to boot. A year ago, a quarter of Poles opposed accepting anyone fleeing the ravages of war in the Middle East; after months of relentless propaganda, 75 percent are now opposed. This year the country has let in only 1,474 asylum seekers, nearly all of them from Russia or Ukraine.

Yet the marchers in Warsaw seem to feel that their country is being overwhelmed. “We don’t want Muslims here,” they cried. “No to Islam.” And “refugees get out.”

Until very recently Poles had never given much thought to Islam beyond occasionally expressing a sense of historical pride that a Polish king, Jan Sobieski, defeated the Turks in a 17th-century battle near Vienna, thus saving Christian Europe from the infidels. This fits a recurrent theme in Polish national mythology: Poland as a rampart of Christianity, the Christ of Nations. Poland, according to this trope, has repeatedly, and heroically, suffered for the sake of others, especially the rest of Christian Europe.

While the Warsaw demonstrators paraded with burning torches, Mr. Kaczynski gave a speech in Krakow expressing a new twist on this familiar narrative: The Poles’ mission now is to save a “sick Europe” from itself. The neo-fascist marchers in Warsaw suggested, as if on cue, how it could be done: “Pure Blood,” read one banner. “White Europe,” another said.

But most Poles couldn’t tell a Muslim or a Buddhist from Jesus. Their animus, which carries Polish nationalism into such an aggressively xenophobic articulation, springs primarily from a deep pool of ethnic-cum-religious hatred, which is indigenous to Poland and has historically been aimed at Jews.