France is coming to terms with the reality that the man it has propelled towards pole position for next year's presidential elections is precisely the kind of political figure whose reforming ambitions have been repeatedly frustrated by popular resistance.

The late surge that established Francois Fillon as favourite to win Sunday's decisive second round of the centre-right primaries seems at odds with the spirit of the age. After Brexit, the election of Donald Trump as president-elect of the US and the steady march of populist parties in much of the West, the former prime minister more obviously represents the ancien regime.

The left-wing Liberation newspaper devoted the front page of its edition covering the first round of the primaries for the conservative Les Republicains party to a caricature of Fillon as Margaret Thatcher.

Certainly, his taste for the shock remedies of slashing public sector jobs, ending cherished trade union privileges and cutting wealth taxes to stimulate growth sits uneasily with the rise of radical politics that has encouraged the far-right Marine Le Pen to believe she could become France's first woman president next May.

"I think he may have to moderate his programme to avoid pushing working-class people towards Le Pen," said Andre, a retired bank worker from Fillon's birthplace of Le Mans, south-west of Paris. "I voted for him and think he may be France's last chance of bringing about necessary economic change.

"But the unions are very powerful and are certain to resist him. The French are always in favour of reforms as long – as they affect only others."

The French left is horrified. "How sad," said Alexis Poulin, a Parisian media worker. "We get Thatcher 35 years later. Fillon won the first round because he embodies social conservatism – the family, work, homeland. His ultra-liberal ideas are 50 years out of date, offering economic policies that have not worked for any country that has tried them.

French Prime Minister, Francois Fillon.
French Prime Minister, Francois Fillon. Reuters

"But if he wins the primaries and campaigns on Le Pen's themes, he could do her a lot of harm."

And Dorine Parravano, from the youth section of the troubled ruling socialist party of the current president Francois Hollande, accused Fillon of seeking to crush the vaunted French "social model", which attaches great importance to employees' rights.

She added: "He wants to get rid of 500,000 public sector workers and this would mean less public service and fewer police and hospital staff, just when we most need them."

Parravano urged ordinary voters people not to "fall into the trap" of being seduced by the populist policies of Ms Le Pen's Front National. "Wherever they have won local elections, they have set out to destroy socially beneficial policies," she said.

Yet Fillon, 62, a civil lawyer's son and career politician whose wife is Welsh, appears to offer France its best hope of avoiding a far-right government. The elimination of former president Nicolas Sarkozy from the first round of the primaries leaves him with only Alain Juppe to beat in Sunday's run-off.

When Marine Le Pen became its president, when she didn't want her father in the party... all these signs symbolise the new Front National.

Juppe, like Fillon a former PM, is seven years older and shows an occasionally grumpy disposition that may have damaged his prospects of winning the nomination as the centre-right Les Republicains' candidate.

Sarkozy, whose own policies – especially towards France's large Muslim population and immigration – veered sharply to the right as he attempted to counter the rise of Le Pen's Front National, has announced his retirement from politics. But he said he would be voting in the second round of the primaries for Fillon, despite past differences, because his programme more closely resembled his own.

The central question that the gathering momentum of a presidential election will answer is the extent to which Fillon's reputation as a safe pair of hands can overcome distrust of the political establishment he represents. His reforming zeal differs only in degree to the same commitment to change expressed by Sarkozy and earlier French leaders, only to be defeated by mass protests and crippling strikes.

Le Pen was quick to welcome both Britain's vote on June 23 to leave the EU and Donald Trump's election to the US presidency.

She has successfully exploited the "ras-de-bol" of ordinary French voters – their state of feeling utterly fed up with mainstream political leaders and parties – and many even seem unconcerned that her party is habitually described as "extreme right wing".

After the Trump victory over Hillary Clinton, Le Pen declared: "It is not the end of the world but the end of a world... it makes possible what has previously been presented as impossible".

Her most significant achievement to date has been the success of her policy of "de-demonisation", a carefully orchestrated campaign to throw off her party's image as a racist, even fascist movement. This impression was formed in large measure by the rabble-rousing declarations of its founder, her father Jean-Marie.

Marine Le Pen
Front National leader Marine Le Pen gestures during a political rally in Frejus, France on 18 September 2016 Jean-Paul Pelissier/File Photo/Reuters

She has profited from being able to present the FN as a "party like any other", the opposite of what her father always wanted. The desire for change, for new answers to the problems of unemployment, immigration and crime, has made her party France's most popular.

"The Front National has changed over years," says Pierre Dumazeau, a senior political writer from Valeurs Actuels, a magazine whose own popularity has reflected the party's growing appeal. "When Marine Le Pen became its president, when she didn't want her father in the party... all these signs symbolise the new Front National. Her right-hand man, Florian Philippot, even came from a left-wing party. The fact that FN is the leading party in France shows a real anger of the French people."

With opinion polls now discredited after getting it wrong in Britain, the USA and now France, almost anything seems possible in the months that remain before the French choose their next president.

The socialists are in serious disarray, with Hollande yet to declare whether he will stand for re-election in the face of record levels of public disapproval. Most observers currently see Le Pen as likely to beat any socialist candidate to a place in the final found.

If that pits her against Fillon in a battle of the right and far-right, the mood of the streets will hold greater sway over the outcome than the analysis of pundits. On some issues, there is relatively little to separate them: both controversially favour rapprochement with the Russian president Vladimir Putin, both oppose gay marriage.

In 2002, the centre-right Jacques Chirac won the grudging support of socialist electors, willing to support him, after their own candidate was eliminated, to stop Marine Le Pen's father reaching the Elysee. The joke ran that large numbers wore nose-pegs to vote Chirac; Le Pen was crushed in the run-off, polling only 18%.

In 2017, it is difficult to see an equivalent outbreak of tactical voting in favour of Fillon. As one French political commentator, Yves Bourdillon, put it after Fillon's emergence as favourite to carry Les Republicains' flag into the campaign: "It's curious. Portrayed as inoffensive [until the first round result became clear], is now a Catholic Thatcherite who devours babies under the full moon."

Colin Randall spent 29 years at The Daily Telegraph as reporter, chief reporter, executive news editor and Paris bureau chief, was executive editor for the launch of The National (UAE) in 2008, and now freelances and edits websites from London and the south of France. He tweets at @salutsunderland