More than 70,000 migrants recently overwhelmed the beachfront border separating Morocco from the Spanish outpost of Ceuta. Sílvia Orriols, mayor of Ripoll and member of the Catalan regional legislature in Barcelona, was not disoriented — she was on familiar ground. By fiercely opposing migration from North Africa, she has turned her small Catalan Alliance into one of the fastest-growing political parties in Europe, with the potential to disrupt Spanish politics on a much larger scale.

Ms. Orriols criticized the lack of an effective police presence in Ceuta and mocked the Spanish state’s priorities. She reminded her followers on X that in 2017, Catalonia’s independence referendum led to a far more severe crackdown by the national police. “They beat Catalans with truncheons and fired rubber bullets at us for voting in a referendum,” she wrote. “Have you seen any truncheons or rubber bullets in Ceuta? It is urgent that we break with this state that allows repeated violations of its borders.”

In the stridency of her anti-immigration position, Ms. Orriols, 41, can sound like the staunchest right-wing nationalist. She wants a moratorium on immigration and has called for deporting all immigrants who have broken the law in Catalonia. She has campaigned to remove halal menus from public schools and argued that Islam is incompatible with Western values. But apart from her attitude toward migration, her views are not particularly conservative. She supports women’s rights and gay rights, and wears a cross but speaks more about Catalonia’s Christian roots than Christianity itself.

For most of the past century, the tradition Ms. Orriols represents — Catalan nationalism — has been left-leaning, opposed to authoritarianism and associated with a cosmopolitan elite in Barcelona. Catalonia was a last redoubt of the second Spanish Republic against the military uprising of Gen. Francisco Franco, which triumphed with the backing of Nazi Germany and Fascist Italy in the late 1930s. But in her unwavering opposition to immigration, Ms. Orriols is fashioning a version of Catalan nationalism that is more populist, more rural and more confrontational, shifting the question from political power to ethnic survival.