As the sun set over the palm trees of the French Riviera and the millionaires’ yachts bobbed in the harbour, crowds of flag-waving supporters were squeezed into a tiny hall in Antibes, cheering a young woman styled by her admirers as a Joan of Arc of the far right.

“What unites us is an unconditional love for France,” Marion Maréchal-Le Pen told an eclectic audience ranging from retired business leaders in smart loafers to heavy-metal fans, poor farmers, trendy teenage girls and people carrying lapdogs with bows in their hair. Many had come for the first time to witness the much-vaunted oratorical skills of France’s youngest MP – and to see how she compared to her grandfather, the gruff former paratrooper Jean-Marie Le Pen, who co-founded the Front National in 1972 and led it to become the most successful far-right party in western Europe.

Maréchal-Le Pen, 25, the Front National’s biggest young star, is at the fore of a crucial new battle in the party’s grassroots rise across France. She is running to lead the southern region of Provence-Alpes-Côte d’Azur in regional elections next month. A historic win is within reach that would cement the party’s drive for local power and boost the presidential ambitions of her aunt and party leader, Marine Le Pen.

For decades, the sun-soaked south of France was the heartland of “Grandpa” Le Pen, who built up an electoral base here in the early 1990s. That party powerbase has now mushroomed: when a record 11 Front National mayors were elected across France last year, five were in towns in this southern region. The Front National’s first two senators are both from the south. The vast Provence-Alpes-Côte d’Azur region has five million people spreading from coastal cities such as Marseille up to Avignon in Vaucluse, taking in Riviera beach resorts and tiny mountain villages. It is one of the richest regions in France, with a vast budget that one local Front National figure pointed out was “bigger than the budgets of some European countries”.

But although the Riviera is synonymous with the eye-watering wealth of figures such as the Russian billionaire Roman Abramovich, who has a chateau near Antibes, the region also has a large disparity between rich and poor and isn’t immune to France’s historically high 11% unemployment rate and economic gloom. Maréchal-Le Pen is on a mission to court what she calls “the France of the forgotten”, running a hardline campaign on identity, immigration and security.

Christian Estrosi is Marion Marechal-Le Pen’s main rival. Photograph: Splash News/Corbis

Behind her campaign looms the shadow of her aunt, who is running to head the north-eastern region of Nord-Pas-de-Calais-Picardie and use it as a springboard for her presidential ambitions in 2017. Marine Le Pen has led a public relations drive to “detoxify” the party and move it away from the racist, jackbooted, antisemitic imagery of the past. But its hardline positions on Islam and immigration remain unchanged. If Marine Le Pen and her niece, as polls predict, manage to win the first two French regions ever to be led by the FN, they hope it will draw a line under the bitter public family feud that has led Marine Le Pen to push her father out of the party amid ongoing courtroom spats.

However, the Riviera battle also provides a platform for the devoutly Catholic Maréchal-Le Pen to showcase how she is even more rightwing and socially conservative than her aunt.

On stage, unlike her grandfather and aunt, famed for their tub-thumping speeches, Maréchal-Le Pen takes the mic off its stand and walks around like a friendly compere, talking in a soft voice and ripping apart her opponents with barbed jokes. She warned that France was under threat of being submerged by immigrants and radical Islam and disappearing completely, making bold references to the “great replacement” theory by the controversial writer, Renaud Camus, who has argued that local French populations will be replaced by newcomers who reproduce faster. “There’s a great replacement of populations at work, let’s be frank,” she said, adding that the most “terrifying symptom” of this was the arrival of refugees.

She asked why fleeing Syrians had not stayed and fought. “In France, when there was a war we fought and our ancestors fought, though many had real reason to flee the Germans.” With an echo of her religious beliefs, she said she could bring France “a sublime resurrection”, to cheers and applause.

Her election leaflets go far: she says she does not want a region that is “black-blanc-beur” – “black, white, Arab”, a term used to hail the multicultural French football team that won the World cup in 1998 – but instead wants a region that is “bleu-blanc-rouge”, the colours of the French flag. She has said she doesn’t want “the Riviera to turn into the favelas”, an allusion to Brazilian shanty towns.

Among the candidates she had chosen to run with her as regional councillors are several hardline figures once kept at arm’s length from the party by her aunt, including Philippe Vardon, a former leader of the small Bloc Identitaire, which is even further to the right than the Front National.

Denise Soler, a former shop assistant and ‘pied-noir’ – the name given to French people born in Algeria who returned to France after independence – said: “I’m not racist but people are worried by the number of Muslims arriving in France.”

Isabelle Houssays, an insurance worker who joined the party last year, impressed by its new, “softer” image, said: “She’s a visionary who wants to save our country.”

Hervé, a telecoms technician in his 50s who used to vote for the left, had come out of curiosity and because he agreed with her drive to allocate benefits for the French above immigrants. He went home saying: “She blew me away.”

Marine and Jean-Marie Le Pen, Marion’s aunt and grandfather respectively. Photograph: Robert Pratta/Reuters

Polls put Maréchal-Le Pen slightly ahead, but the battle is extremely close. It has become a deeply personal fight with her main opponent, the debonair MP and mayor of Nice, Christian Estrosi, from former president Nicolas Sarkozy’s right-wing Les Républicains party. Estrosi, a former motor bike champion and government minister, 60, is already seen as on the hard right of his party, increasing security measures in Nice and banning “youyous” or north African cries of joy from weddings at his town hall.

Earlier this year he publicly complained of a “fifth column” of Islamists in France and a “world war three” against the country’s Christian roots. He has attacked Maréchal-Le Pen as “the most dangerous of the three Le Pens”, slamming her for her “extremism” and her cosseted upbringing at her grandfather’s posh manor estate outside Paris.

With the left divided and trailing far behind, several surprising but key figures who would not normally vote right have backed Estrosi in a bid to keep out the FN. Serge Klarsfeld, the celebrated French lawyer and Nazi-hunter who was hidden from Nazis in Nice as a child during the occupation, has urged people to vote for Estrosi to block what he said was a “racist, xenophobic and antisemitic” party.

Mourad Boudjellal, president of Toulon rugby club, also backed Estrosi against what he called Front National “blackshirts”. It remains to be seen whether the Socialists, expected to come third, will pull out of the second round to allow Estrosi a clear run against Maréchal-Le Pen. That question remains controversial on the left.

Meanwhile, the local Front National grassroots were hailing the party’s progress. “Ten years ago our vote was a protest vote; now people adhere to our ideas as a real alternative,” said Lionel Tivoli, a young local councillor and IT technician who joined the party at 18. “I’m very far from being racist, but I don’t accept that people who come here don’t adapt to our customs and impose theirs.”

At the Antibes rally, some who had never voted Front National said it might be a “necessary evil” to shake up French politics. Christian Noirel, who used to run a small public works company and until now had been sceptical of the Front National’s economic policy and drive to leave the euro, said: “France needs a strong opposition party that can wake other politicians up.”