The beleaguered rightwing French presidential candidate François Fillon has used a speech at a Paris rally to hit back at claims that his wife was paid €500,000 over eight years for a fake job as a parliamentary assistant, warning: “Leave my wife out of the election.”

As his British wife, Penelope, sat in the front row in her first public appearance since the scandal broke last week, he said: “I want to tell Penelope that I love her and that I will never forgive those who tried to throw us to the wolves.” The crowd chanted “Penelope, Penelope!”

Fillon said: “If someone wants to attack me they should attack me straight on, but leave my wife out of this political debate.”

“They are trying to take me down, through Penelope,” he told the crowd of mostly older supporters in a Paris hall, who interrupted his speech with shouts of “Fillon, president!”

But just as Fillon attempted to kickstart his flagging presidential campaign, he faced fresh questions over alleged misuse of public funds.

The French investigative website Mediapart and the Journal du Dimanche claimed that between 2005 and 2007 Fillon had pocketed money from a kitty of funds earmarked for paying assistants in the French senate.

For four years, a French legal investigation has been under way into an allegedly widespread practice in which funds left over from senators’ allowances for paying assistants were put into a kitty and then a part of the money was siphoned off to the senators.

Five people, including three senators, have been placed under official investigation over the alleged practice. The ongoing legal inquiry is into alleged misuse of public funds across the senate after 2009.

But Mediapart and Journal du Dimanche allege that Fillon was involved in the same type of practice when he was a member of the senate between 2005 and 2007.

Mediapart claimed he had “siphoned off” about €25,000 (£21,000) from funds earmarked for assistants in the French upper house.

The Journal du Dimanche alleged he had written seven cheques to himself between 2005 and 2007 for “a total of around €21,000”.

A spokesperson for Fillon declined to comment to the news agency AFP, saying only that a judicial process was under way. Fillon’s entourage told the Journal du Dimanche the issue was an “old story”. Fillon’s ally, the senate leader, Gérard Larcher, told French radio that leftover funds from senate assistants’ budgets were generally pooled in order to be reallocated for political work.

The fresh claims add to growing pressure on Fillon, the candidate for the rightwing Republican party, who had styled himself as sleaze-free and had been considered a frontrunner for next April and May’s election. Recent polls have indicated support for Fillon falling slightly. He is slightly behind the far-right Front National’s Marine Le Pen, with Emmanuel Macron, the maverick independent centrist, breathing down his neck.

Last week, state financial prosecutors opened a preliminary investigation into possible misuse of public funds to determine whether Penelope Fillon was paid a very generous salary from public funds for a job she allegedly didn’t carry out. Prosecutors are also investigating whether a high salary paid to her from a magazine owned by a billionaire friend of Fillon amounted to “misuse of company assets”.

Fillon has denied any wrongdoing, insisting the jobs carried out by his wife were “real”.

On television last week, Fillon rebutted the allegations against his wife and revealed that he had also paid two of his children from state funds for work for him when he was a senator, adding he hired them for their “competence” as lawyers. But his children were students at the time and did not qualify as lawyers until a later date.

Fillon told the Journal du Dimanche that there were “forces at work” to “weaken” his candidacy or stop him from running.

“It is a terrible plot, but I am sure that the justice system will not allow itself to be exploited by these defamatory allegations,” he said.

The issue is potentially so damaging because Fillon’s austerity plan for France hangs on his own carefully crafted reputation for righteousness. It will be much harder for Fillon to convince a cash-strapped electorate of his controversial plans to slash 500,000 public-sector jobs and make state workers put in more hours for less pay if questions persist about his family’s privileged access to jobs paid for by their taxes.