“This is the new normal,” said Flemming Rose, the foreign editor at the Danish newspaper Jyllands-Posten. “It is a nightmare for traditional political elites and also for Brussels.”

The platform of France’s National Front promotes traditional right-wing causes like law and order and tight controls on immigration but reads in parts like a leftist manifesto. It accuses “big bosses” of promoting open borders so they can import cheap labor to drive down wages. It rails against globalization as a threat to French language and culture, and it opposes any rise in the retirement age or cuts in pensions.

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Similarly, in the Netherlands, Geert Wilders, the anti-Islam leader of the Party for Freedom, has mixed attacks on immigration with promises to defend welfare entitlements. “He is the only one who says we don’t have to cut anything,” said Chris Aalberts, a scholar at Erasmus University in Rotterdam and author of a book based on interviews with Mr. Wilders’s supporters. “This is a popular message.”

Mr. Wilders, who has police protection because of death threats from Muslim extremists, is best known for his attacks on Islam and demands that the Quran be banned. These issues, Mr. Aalberts said, “are not a big vote winner,” but they help set him apart from deeply unpopular centrist politicians who talk mainly about budget cuts. The success of populist parties, Mr. Aalberts added, “is more about the collapse of the center than the attractiveness of the alternatives.”

Pia Kjaersgaard, the pioneer of a trend now being felt across Europe, set up the Danish People’s Party in 1995 and began shaping what critics dismissed as a rabble of misfits and racists into a highly disciplined, effective and even mainstream political force.

Ms. Kjaersgaard, a former social worker who led the party until last year, said she rigorously screened membership lists, weeding out anyone with views that might comfort critics who see her party as extremist. She said she had urged a similar cleansing of the ranks in Sweden’s anti-immigration and anti-Brussels movement, the Swedish Democrats, whose early leaders included a former activist in the Nordic Reich Party.