Last month, Sophie-Laurence Roy, a conservative Paris lawyer with roots in Burgundy, decided to cross the political dividing line that defined postwar France and dedicate herself to a nationalist, far-right political movement that seems poised to dominate parliamentary elections on Sunday.

“I realized I would reproach myself for the rest of my life if I did not offer my services to the great movement of change that is the National Rally,” she said as she ate a sausage of pork intestines in a cafe in Chablis, the northern Burgundy town known for its fine white wine. “It was now or never.”

So, on June 9, Ms. Roy, 68, deserted her longtime center-right political family, the Republicans, who trace their beliefs to the wartime hero Charles de Gaulle, to support Marine Le Pen’s far-right party whose quasi-fascist roots lie with the collaborationist Vichy regime against which De Gaulle fought to liberate France.

How could she cross such a chasm? “My problem is not the past, it’s tomorrow,” Ms. Roy, who is now a candidate allied with the National Rally in the largest constituency of the Yonne district, which includes Chablis, said dismissively. “People are suffering.”

Some 9.3 million people voted for the National Rally in the first round of the election last weekend, more than double the 4.2 million in the first round of parliamentary elections in 2022. Spread across most regions in France, they included workers and pensioners, the young and the old, women and men. Tired of the status quo, they came together to roll the dice for change.

Now, Ms. Le Pen’s party — one that has softened its image and smoothed its message, but retained a core anti-immigrant and euro-skeptic creed — seems set to become the largest in France after the second round of voting, even if it now appears unlikely to win an absolute majority.

To say a taboo has fallen against voting for the far right is insufficient; it has disintegrated in a tidal wave of National Rally support.

Tensions have risen across the country as a result. The Interior Ministry has announced that 30,000 police officers will be deployed on Sunday “to prevent the risk of disorder.”

Residents in this sparsely populated region of France — the Yonne district in northwestern Burgundy has only about 335,000 inhabitants — describe what is happening to their community as “desertification,” by which they mean an emptying out of services, and of their lives.

Schools close. Train stations close. Post offices close. Doctors and dentists leave. Cafés and small convenience stores close, squeezed by megastores. People need to go further for services, jobs and food. Many travel in their old cars but are encouraged by the authorities to switch to electric cars, which are priced way beyond their means.

At the same time, since the war in Ukraine, gas and electricity bills have shot up, leading some to switch off their heating last winter. They feel invisible and only just get by; and on their televisions they see President Emmanuel Macron explaining the critical importance of such abstract policies as European “strategic autonomy.” It is not their concern.

Along comes the National Rally, saying its focus is on people, not ideas, the purchasing power of people above all.

“My party is anchored in this territory, it is not, like our president, trying to give moral lessons to the whole world,” Ms. Roy said.

The pervasive unease is not always easy to grasp. The beautiful rolling hills of the Yonne, the rows of Chablis vines on the escarpment above the Serein river, and the golden fields of wheat in the afternoon sunlight do not suggest turmoil. Yet discontent brews on French soil more than is readily apparent.

In the main square in Chablis, as in most French towns and villages, stands a monument to the toll of war. “Chablis to its Glorious Dead,” reads the inscription above a list of 13 dead in the 1870-71 war with Germany, 76 dead in World War I, four dead in World War II, two dead in the war in Indochina, and one dead in the Algerian war.

Above the monument flies the French flag and the blue-and-gold European Union flag, a symbol of the commitment to ending war through European integration, the process that removed borders and gave France its ideological framework and moral foundation from 1945 onward.

That framework and that foundation are now wobbly.

The National Rally wants to return power to the nation. It wants to tighten the open internal borders of the European Union to slow migration. It is ready to mythologize national greatness, in a lower key than the 20th-century merchants of hysteria who plunged the continent into war, but with the same dizzying, scapegoat-identifying intent.

The ground is fertile for such appeals. “Our French heartland has the feeling of being forgotten,” said André Villiers, a centrist allied to the party of Mr. Macron — and Ms. Roy’s opponent in Sunday’s runoff. “What you see here in the National Rally surge is anger and alienation.”

Mr. Villiers, 69, the incumbent and a lawmaker in the National Assembly since 2017, was seated in a cafe in the beautiful town of Vézelay, about 30 miles south of Chablis.

Nearby was the 1,000-year-old Vézelay Abbey, said to contain relics of Mary Magdalene. It has long been an important place of pilgrimage associated with miracles. Mr. Villiers may need one, given the results in the first round of voting in his district.

“Macron is at his low point,” he said. “People want him gone, his page is turned, and that does not help.”

In the first round of voting, Mr. Villiers took 29.3 percent of the vote to Ms. Roy’s 44.5 percent. The left-wing candidate, who has now dropped out and urged his supporters to use their votes to stop a National Rally victory, took 19.5 percent. Ms. Roy is the favorite, although the result will likely be close.

In Avallon, near Vézelay, I met Pascal Tissier, 64, who recently retired after working as a traveling salesman. He voted for Mr. Villiers in the first round, “but now I am tempted to vote for the National Rally, because something that has been heating up for a very long time is happening.”

“What,” I asked?

“I cut the heat in my house a few months ago, because the bill had become impossible,” he said. “Bus services have been eliminated. I have to travel 45 minutes to Tonnerre, because the tax office here closed. It’s simple: People feel belittled by Macron.”

Life has become harder in other ways. His father is 90 and lives alone in Rouvray, 12 miles away. Every two days, Mr. Tissier brings him food, because the one remaining food store near his father closed a few months ago. The local doctor retired this year.

“The government pays no attention to all this,” Mr. Tissier said. “It’s bizarre.”

Into this sort of vacuum, across the country, the National Rally stepped. The party says it has shed its xenophobic, bigoted past, but every now and again, including in the Yonne, the old tropes resurface, rising like Dr. Strangelove’s gloved arm.

This past week, Daniel Grenon, the incumbent and a National Rally candidate in another Yonne constituency, declared that “North Africans do not have a place in high office.” He was apparently referring to French citizens of North African descent or dual nationality. The secretary of the Yonne Socialist Party immediately sued him for incitement to hatred and discrimination.

Jordan Bardella, the smooth 28-year-old leader of the National Rally in the election campaign, who has sought to distance the party from overt prejudice, said in a television interview that Mr. Grenon’s statement was “abject.” Asked whether he would continue to support the candidate, Mr. Bardella said that Mr. Grenon, if re-elected, would no longer sit with the National Rally group in the National Assembly.

Another National Rally lawmaker and candidate, Roger Chudeau, infuriated Ms. Le Pen last week by saying that a former education minister, Najat Vallaud-Belkacem, of dual French and Moroccan nationality, had “destroyed middle school” and that ministry positions should go to “Franco-French people, and that’s final.”

“I am shocked by our colleague Chudeau,” Ms. Le Pen said. Yet the supposed dilution of a Frenchness by immigrants remains at the heart of her party’s message.

Mr. Villiers believes the National Rally’s threat to the Republic remains real. “The wick between us and the bomb is short,” he said. “We know how this begins and how it ends. I will fight to the end.”

He called Ms. Roy’s leap from the Republicans to the National Rally “a grave moral abandonment.”

In Chablis, a city of winemakers who depend on exports for much of their revenue, the ascendant National Rally message feels worrying to some. “Closing borders does not work for us,” said Damien Leclerc, the director-general of a big wine cooperative, La Chablisienne. Last year, 62 percent of its $67 million in sales was from exports.

Winemakers depend on the outside world in other ways, too. “We need migrant laborers for all the manual work,” Mr. Leclerc said. “We need them to do weeding, to prune the vines, to trellis the vines, jobs the French don’t generally want to do.”

Ridial Diamé, 38, a Senegalese laborer, was about to break for lunch when I found him in the Chablis vineyards on a steep hillside. It was noon; he had started work in the early morning, mainly weeding on an estate called Domaine Goulley where chemical sprays are not used. A Muslim with a wife and two children in Senegal, he previously worked in Spain and is on a temporary contract in Chablis.

“It’s pretty good work,” he said. “I do a 35-hour week at about $13 an hour; we have three days off. I’ll stay as long as I can.”

What did he think of the anti-immigrant policies of the National Rally?

“It’s very funny,” he said. “The French don’t want to do these jobs, so we do them. And then they say they don’t want us!”

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