PARIS — After a week in which she and her far-right National Front party seemed ascendant, Marine Le Pen is heading into Sunday’s second round of French regional elections facing a new and more challenging political equation that will test the appeal of her nationalist, anti-immigrant message.
The National Front was the clear winner in the first round last week, stunning the governing Socialists as well as the mainstream conservative party, the Republicans, and raising expectations that Ms. Le Pen would emerge from the second round with victories in at least two regions. But the latest polling suggests that she could be in for a tough battle both in the northern region around Lille, where she is on the ballot herself, and in a southern region around Nice being contested by her niece Marion Marechal Le Pen.
The difference: The Socialist Party candidates, who finished third in the first round of both those races, withdrew this week in order to leave the field clear for the Republicans. Their logic is that it is better to unify the anti-National Front vote, despite the enmity between the Socialists and the Republicans, than to allow Ms. Le Pen to split the vote of the more traditional parties.
Ms. Le Pen and her associates can correctly boast that theirs is the “first party of France,” as they did at a rally here on Thursday night: The National Front was easily the top vote-getter among the three leading parties in the first round of the current elections. But when one of the mainstream parties withdraws, the election comes down to the National Front versus the anti-National Front vote. And judging by polls and previous election results — in local elections in March, the National Front was beaten time after time in head-to-head matchups with mainstream conservatives in the second round — the anti-National Front vote can frequently be a substantial majority.
In response to the decision by the Socialists to leave the field in favor of the Republicans, the party of former President Nicolas Sarkozy, Ms. Le Pen has been lashing out at what she called “the political mafia” formed by her opponents.
At the rally on Thursday night, in front of hundreds of cheering, flag-waving supporters at the Salle Wagram theater here, Ms. Le Pen, between boasts and mocking attacks on her opponents, acknowledged that success for her party was not assured.
“There remains a second round, and nothing is guaranteed,” she told the roaring crowd, as strobe lights flashed in the packed hall and music pounded. But she immediately reassured them by adding, “We know that the moment of change has been initiated.”
Her supporters loved it when Ms. Le Pen called the Socialist prime minister, Manuel Valls — who for weeks has been suggesting a coalition with rivals to block the National Front on Election Day — a “junior-grade braggart,” using a Spanish-origin word, “matamor,” or show-off, that subtly evoked his birth in Spain. It was a nod to her party’s anti-immigration stance.
The party has been riding high for weeks, its expectations substantially bolstered in a first round of voting last Sunday: the National Front came out largely on top in both the northern and southern regions, winning over 40 percent of the vote, way ahead of its nearest rivals, and shocking France’s political and media establishment and underscoring Ms. Le Pen’s credibility as a presidential candidate in 2017. “The old world is in the process of disappearing, right in front of our eyes,” Ms. Le Pen said Thursday night.
All week, the country’s news media has been replete with lamentations about the possibility of a far-right party, rooted in wartime collaboration and now surfing on an anti-immigrant wave, taking over large sections of France. A victory by the Front would also be something of a first in Europe, commentators have pointed out, because extremist parties have been forced to seek alliances with mainstream parties elsewhere on the Continent.
Regions of France that have traditionally resisted the party’s appeals — the southwest, the Alps, central areas around the Loire — appeared to have fallen under its spell in last Sunday’s voting, with the Front increasing its support substantially in all those places. Pollsters said the Paris terrorist attacks of Nov. 13 had bolstered the party, with its anti-immigrant message finding new resonance in a fearful electorate.
Still, the apprehension of its opponents may be overblown. Two polls published Thursday showed that even in the northern region where the gravel-voiced Ms. Le Pen herself is running, her mainstream conservative opponent could hold a slight lead.
Similarly, in the south, where her niece is running, three polls this week have placed the conservative mayor of Nice, Christian Estrosi, on top in a two-party race after being trounced by more than 14 points in the three-party matchup on Sunday.
Ms. Le Pen bitterly acknowledged the changed dynamic on Thursday night, pleading with voters to “go beyond the right-left split and vote for the Front.”
In a speech, she said that her opponents were “operetta resistance fighters” and that the “curtain had fallen” on the “tragic farce” of mainstream party governance.
The careful French she deploys, classically rooted, and the way she uses references to France’s cultural heritage — she invoked a famous 19th-century painting, Théodore Géricault’s “Raft of the Medusa” — are anything but populist. Instead, they evoke what she called “the millennial values which have created France,” a cultural legacy of centuries that the National Front insists is now threatened by what it calls “the migratory submersion.”
Even if the party were to win in the two regions where Ms. Le Pen and her niece are running, the effect would be more symbolic than practical. Presidents and assemblies of the country’s 13 metropolitan regions control little beyond physical infrastructure like ports and airports, as well as upkeep for schools and other secondary matters.
But the Front’s opponents consider handing over to her even limited powers intolerable. Mr. Valls said on Friday that an ascendant Front could lead to “civil war” in France, and that its program was a “swindle.”
Front voters at Thursday’s rally expressed frustration that once again, the two mainstream parties appeared determined to keep Ms. Le Pen’s from taking power. “They do it every time. Every time. But we’ll see what happens Sunday,” said Nellie Viaro, a Front municipal council member from the Paris region.
“All these parties are uniting against Marine Le Pen with marriages that defy nature,” said Geneviève Pageard, 76, a resident of Paris. “Those people are just drunk with ideology.”
Because of an editing error, an earlier version of this article misstated the National Front’s share of the vote in the first round of regional elections. The party took 40 percent of the vote in the northern and southern regions; it did not take 40 percent nationwide. (The National Front won 28 percent nationwide in the first round.)