CHASSIEU, France — A patriotic sea of supporters, waving red, white and blue flags, greeted the newly ascendant presidential hopeful François Fillon this week at a soulless conference center in this suburb of Lyon.
Mr. Fillon described radical Islam as a “totalitarianism like the Nazis” to the cheers of an enthusiastic crowd, adding that France would need Russia’s help to fight it. Catholics, Protestants and Jews “don’t denounce the values of the Republic,” he thundered — unlike the faithful of a certain other religion.
“We’ve got to reduce immigration to its strict minimum,” he said. “Our country is not a sum of communities, it is an identity!”
In a year when nativist politics have become the ticket to electoral victory, Mr. Fillon, 62, a dark-suited, stern-faced former prime minister has managed to successfully ride the same nationalist and xenophobic currents as that have pushed politicians in Britain and the United States to victory.
For months, Mr. Fillon polled third and even fourth among presidential contenders in France and was largely dismissed. But his defense of French values and identity has suddenly made him the front-runner as France’s right-center Republican Party prepares to vote Sunday in a runoff to choose its standard-bearer in the 2017 elections — and quite possibly the next president of France.
That strategy has not only vaulted Mr. Fillon to the front of the pack, surprising the French news media, pundits and politicians. It has also has shifted the playing field for French conservatives far to the right, snug alongside the far-right National Front, led by Marine Le Pen, who may be his strongest challenger next year.
The two are now competing for some of the same voters, but the xenophobic, anti-immigrant National Front is not regarded as respectable by many middle-class French people, particularly Catholics.
Mr. Fillon, on the other hand, has positioned himself as a staunch defender of French values, vowing to restore authority, honor the Roman Catholic Church and exert “strict administrative control” over Islam.
Such appeals were met with fervent cheers during his campaign rally here by supporters who agreed that French identity and French values were under threat from both Islam and the secular left.
If those themes sound familiar to the ones that already shaped pivotal votes in Britain and the United States this year, it is because they are. And Mr. Fillon, an unsmiling political veteran, has used them to remarkable effect in France to change his political fortunes.
Few analysts give France’s Socialists and their unpopular president, François Hollande, a strong chance in the spring. But Mr. Fillon, with his dark-eyed frown, suggests a return to authority, and the rally crowd of about 5,000 here loved it.
“He really respects French values, and that is very, very important to us,” said Vincent Robert, a man in late middle-age who said he was a retired artisan.
“There is uprightness and frankness,” Mr. Robert said. “And incredible enthusiasm for him in the middle class.”
His friend Bruno Perrin, a retired management consultant, was equally in thrall. “It’s his defense of values — Christianity, the family, our traditions — that is what we like,” he said.
Mr. Fillon’s supporters say he is now the candidate best placed to bar Ms. Le Pen’s path to victory.
Analysts, however, warn that Mr. Fillon’s proposed austerity cure for France’s economic stagnation — cutting 500,000 Civil Service positions and 100 billion euros (about $105 billion) in spending — could be just the thing to push frightened voters into the arms of the National Front leader.
Still, parishioners at Paris’s solidly bourgeois Sainte-Trinité church, a hulking 19th-century neo-Renaissance monument that looms over a busy square, were in no doubt that Mr. Fillon was the one.
“He’s got a sense of honesty and the family,” said one churchgoer, Cécile Despointes, after midday Mass this week. “A man of courage and perseverance.”
It is a surprising turnabout for Mr. Fillon, who just weeks ago was best known as the unsuccessful prime minister from 2007-2012 of a failed French president, Nicolas Sarkozy, who mocked him behind his back.
“Courage, Fillon!” Mr. Sarkozy would say derisively, Patrick Buisson, a far-right journalist and one-time adviser to Mr. Sarkozy, wrote in his recent book “The People’s Cause.”
Then, Mr. Fillon trumped both Mr. Sarkozy — humiliating him in Sunday’s round of voting — and the pollsters.
He came out on top in 87 of France’s 101 départements, or districts, with 44 percent of the vote, 16 points ahead of the rival he faces Sunday, another former prime minister, Alain Juppé. Mr. Fillon won in Paris and in rural France.
Mr. Juppé’s more moderate agenda — fewer cuts in the Civil Service and public spending, less accommodation of Russia, friendlier to gay couples — attracted left-leaning voters, which in turn disturbed the right’s traditional electorate.
“It bothered me that so many people on the left voted for him,” Michel Carron, a rural official who attended the rally here, said of Mr. Juppé.
Mr. Fillon “defends our values,” he said. “We’re in a sort of despair, and he makes us live,” Mr. Carron said, adding that he knows National Front sympathizers who voted for him.
As in the United States, the pollsters had failed to detect a hidden yearning for the restoration of a mythical older France, rural and dominated by the values of a conservative Roman Catholic Church.
Mr. Fillon and Alain Juppé, the two candidates for the right-wing Republican party, during a debate in Paris on Thursday. Credit Pool photo by Eric Feferberg
“The media, television, they are out of phase with the population, which is fed up with being called xenophobic and racist,” said Ms. Despointes, after Mass outside Sainte-Trinité in Paris.
Mr. Fillon is not keen on social changes like same-sex marriage and chafes at being the “vassal” of America, as he put it in his speech here. The crowd at the rally was almost all white.
“He’ll help us to find our identity again,” said Jacqueline Badoux, a retired teacher who attended the rally.
“In the fight against Islamic State, he will have the authority,” said Ms. Badoux’s husband, Bernard, also a retired teacher, who approved mightily of Mr. Fillon’s performance here.
The very qualities the mercurial big-city Mr. Sarkozy made fun of, according to Mr. Buisson — Mr. Fillon’s rural origins and ponderousness, “his legendary caution and his silences” — earned him favor with the crowd here.
“This country is the daughter of Christianity, as well as the Enlightenment,” Mr. Fillon told the crowd outside Lyon. “I will put the family back at the heart of all public policy.” The crowd roared.
Catholics are gravitating toward him because “for the essential values — family, nation, identity — Fillon incarnates them with a certain tranquillity,” said the Rev. Pierre-Hervé Grosjean, a Versailles priest who writes an influential Catholic blog.
“There’s a desire to break with the spirit of May ’68 and go back to the values of family and education, to a France that is not ashamed of its roots,” said Father Grosjean, referring to a period when thousands of demonstrators took to the streets protesting France’s old-world order. For Catholics, “he captures this alliance between conviction and realism,” he said.
At the rally, Mr. Fillon promised to bring back school uniforms, restore respect in schools and stand up to America.
Mr. Fillon is reminiscent of President-elect Donald J. Trump in a significant way: He is friendlier toward Russia and its leader, Vladimir V. Putin.
Indeed, the French news media has detailed his numerous ties to Mr. Putin  — he has stayed at Mr. Putin’s residence at Sochi, according to the newspaper Le Monde, and played billiards with him.
He even received a bottle of wine from the Russian leader on the death of his mother, according to the newsmagazine L’Express , which described Mr. Fillon as the French politician who “appreciates” Mr. Putin the most.
Such concerns, it appears, did little to damage Mr. Trump’s candidacy in the United States. Similarly, they may be just as easy to overlook for supporters of Mr. Fillon like Ms. Despointes.
“He’s a man of conviction,” she said outside Sainte-Trinité. “For people who have moral values.”
ADAM NOSSITERNOV. 25, 2016