Once again, François Fillon, the French right’s scandal-hit presidential candidate, has refused to quit the election race. Badly wounded and with his party, Les Républicains, bewildered and divided, it remains to be seen whether he can reach the finish line.

Fillon, who once styled himself a sleaze-free “Mr Clean”, now finds himself at the centre of a full judicial investigation into misuse of public funds and a list of other potential offences. He announced on Wednesday that judges had summoned him to press charges later this month.

Fillon is alleged to have paid his British wife, Penelope, at least €680,000 (£577,000) of taxpayers’ money for a suspected fake parliamentary assistant job spanning 15 years. He is also being investigated over giving his children highly paid, allegedly fake jobs from state funds when they were still students. Although French lawmakers are allowed to employ family members, it is unclear what work Penelope did. Fillon denies that he or any of his family members ever broke the law.

François Fillon with his wife, Penelope, at a rally in Paris in January. Photograph: Pascal Rossignol/Reuters

By refusing to quit and accusing the French state of a political “assassination” attempt not just against him but the whole democratic process, and by urging his wronged voters to “resist”, Fillon was playing his last card.

Five weeks after the allegations first broke, on-the-stump campaigning has become damagingly difficult for Fillon, who has appeared to retreat into a bunker. Although on-message party workers applaud him at carefully staged rallies, when he ventures outside those confines, he is inevitably met by protesters banging saucepans and shouting “crook!”

Activists protest against corruption in politics. Photograph: Francois Mori/AP

In recent days he seems to have decided to avoid mingling with the public at all costs, instead holding closed meetings or round-table events with cherry-picked voters, and travelling by car not train.

Only three months ago, Fillon was basking in the position of presidential favourite, portraying himself as the only clean pair of hands capable of leading the country through the blood, sweat and tears of austerity. Now he is weakened and permanently on the defensive. Even in his well-heeled Paris parliamentary constituency, die-hard supporters complain that his campaign now looks “pretty damn difficult” and that the right, once seen as incapable of losing this election, faces being knocked out before the final round.

François Fillon (centre left) has lunch with his campaign team. Photograph: Yoan Valat/EPA

Most polls show Marine Le Pen of the far-right Front National and the independent centrist Emmanuel Macron qualifying from the first round in April.

Fillon’s last-ditch strategy is to convince just enough core rightwing voters that he is the victim of a terrible plot against him by the justice system, the government and the media. He hopes to mobilise voter outrage against the system.

Francois Fillon with Nicolas Sarkozy in happier times in 2010. Photograph: Charles Platiau/Reuters

He has taken the advice of the former president Nicolas Sarkozy, whom he once mocked for having to deal with judicial woes. Sarkozy’s suggestion was to grab headlines every day with radical, hardline policy. Fillon has tried this – first he said the legal age of responsibility should be lowered to 16, then he said he would arm all municipal police. But each time he has been drowned out by fresh twists in the legal investigations.

Protests before a presidential campaign rally in Nantes by the National Front’s Marine Le Pen. Photograph: Jeff J Mitchell/Getty Images

With every intervention, Fillon has cranked up his language to stir outrage on the right. On Wednesday, he said the judicial investigation was the state attempting to “muzzle” rightwing voters and steal the election. Last weekend, he warned – after protests against Front National supporters in Nantes – that the government was allowing tensions to fester and that the country was in “a quasi civil war situation”.

“Really, a ‘near civil war’?” shot back Jean-Jacques Urvoas, the Socialist justice minister. “Before he was talking about an ‘institutional coup d’etat’, what will he say next? ‘An extermination of the programmes’? ‘A Holocaust of the candidates’?”

While Fillion addressed his legal woes, his rivals stopped in on Paris international agricultural show. Photograph: Aurelien Morissard/IP3/Getty Images

The fact that Fillon was dramatically forced to postpone an appearance at the Paris farm show to address his legal woes is deeply symbolic. Shaking hands, patting cows and tasting sausage at the annual event is regarded as an unmissable event in the political calendar, allowing candidates to set out their attachment to rural France. Fillon needs to show he is still able to go out into a crowd and chat with the average voter.

Jacques Chirac would famously loiter at the farm fair for 10 hours, sipping beer. It now falls to Fillon, who has styled himself as a country gentleman and lives in a 12th-century rural chateau, to prove that he is not disconnected from the voters.