GUERBIGNY, France--Even before votes are counted from Sunday's local elections, Marine Le Pen's National Front has scored a victory: For the first time, the once-fringe party will field candidates in nearly every constituency across the country--more than any other political group.
The growth underscores the party's success in expanding its base. Ms. Le Pen has toned down its far-right rhetoric, expelled activists for racist comments and latched on to public anger toward mainstream parties and the European Union, which many voters see as responsible for France's economic ills.
The surge is on display in the Somme district in northern France, where elementary-school teacher Yves Dupille is among thousands nationwide joining or returning to the anti-immigration, anti-EU party.
Mr. Dupille left the National Front 16 years ago after a period of infighting, then rejoined just over a year ago and became a town councilor in Amiens in March 2014. On Sunday he's targeting the next rung, seeking to become a representative in the wider Somme region, one of the country's 101 départements.
During a National Front meeting in the town hall of Guerbigny, Mr. Dupille said he had seen little chance of the party gaining power under the leadership of Ms. Le Pen's father, Jean Marie Le Pen. But the transformation under the daughter convinced him that is now possible.
"Marine Le Pen has changed how people see the National Front," Mr. Dupille said. "We have never governed--it is time we had our chance."
Nationwide, the party is running in 1,909 of 2,054 constituencies. That requires mobilizing over 7,500 party activists, as each ticket must have a male-female pairing and two deputies.
Turnout at the elections for such assembly elections is typically low, but the candidates provide a snapshot of the support base of the different national parties.
Among the National Front's candidates in the Somme alone are retirees, students, blue- and white-collar workers, civil servants, an engineer, a lawyer and a former Socialist party activist.
The rise of the National Front is already upsetting the political balance in France. The ruling Socialists of President François Hollande are lumbered with a poor record of high unemployment and hobbled by a split over economic policy. The center-right UMP, headed by former President Nicolas Sarkozy, is rebranding itself after a disappointing showing in EU elections last year.
While many of the National Front candidates will get knocked out by alliances between other parties in a second round of voting March 27, the party is still expected to make gains.
An March 11-13 Ipsos survey of 1,473 people showed the National Front would take 30% of the first-round vote Sunday, followed by the UMP and centrists with a combined 29% and the Socialist Party with 19%.
That is similar to polls before EU elections last May in which the National Front took the biggest share of French vote.
That could give the National Front a say in a wide range of policies from building schools and maintaining roads, to paying unemployment and disability benefits. Such influence will be crucial in building momentum ahead of legislative and presidential elections in 2017.
"The National Front will be able to create a network of people rooted at a local level and demonstrate greater influence in the life of the nation," said Jean-Daniel Lévy, a political analyst at polling company Harris Interactive.
That would strengthen Ms. Le Pen's claim that she now heads France's leading political party.
Prime Minister Manuel Valls has said that a big win for the National Front would be extremely serious for France and its international image. "I'm scared for my country. I'm scared it will be shattered by the National Front," Mr. Valls said in a television interview this month.
Analysis of data provided by the Interior Ministry suggests Ms. Le Pen's party is broadening its appeal. Nearly 19% of National Front candidates describe themselves as workers in the private sector and more than 5% are civil servants. There also are 14 doctors and as many lawyers running on National Front tickets.
The party also has a strong youth following. More than 15% of candidates are 30 or under, a figure that falls to 4.8% for the Socialist Party and 5.3% for the UMP. The youngest candidate running in the elections--18-year-old Noémie Clément--is on the National Front ticket. The party is also fielding the oldest candidate, 91-year-old Françoise Sipière.
At the meeting in Guerbigny, 22-year-old Eric Richermoz, who joined the National Front two years ago and is running for the first time, said France's youth have given up on other parties.
"Youth are the first to be hit by decades of globalization and European federalization, which have created mass unemployment," he said.
The party has also succeeded in recruiting among professions typically loyal to far-left and Communist parties: More than half of the 255 candidates describing themselves as blue-collar are running on the National Front ticket.
"Working class people don't vote Socialist anymore," said Mr. Dupille.
Write to William Horobin at William.Horobin@wsj.com