François Fillon, shown in Paris on March 14, has refused to quit the French presidential race, despite mounting troubles. (Christian Hartmann/Reuters)

François Fillon, the leading French conservative candidate for president, was formally placed under investigation Tuesday on suspicion of embezzling and misusing public funds in the latest blow to a struggling campaign he refuses to abandon.

Fillon, a former prime minister and self-styled family-values conservative, was once considered the easy favorite to win France’s presidential election this spring. But then came the accusations — first published in the satirical newspaper Le Canard Enchainé — that his Welsh-born wife and two of his children had received hundreds of thousands of euros in taxpayer money as compensation for jobs they never actually did.

For a candidate who had promised fiscal responsibility, the elimination of unnecessary public spending and the slashing of as many as 500,000 public-sector jobs, the accusations have proved devastating. His once unshakable approval ratings have plummeted in the wake of the scandal, and many in Fillon’s party, the center-right Republicans, have requested that he step aside for a more viable candidate.

Despite these warning signs — not to mention the rising popularity of the far-right National Front, led by Marine Le Pen — Fillon has refused to quit the race, insisting that the scandal is an attempted “political assassination” with little basis in reality.

“I will not yield. I will not surrender. I will not withdraw,” he said at a news conference earlier this month.

The formal investigation is a step toward charges and a potential trial, though not all such investigations end up in court.

Fillon’s troubles deepened as more details related to the scandal emerged this week, tarnishing his carefully cultivated image as a sober and sensible custodian of France’s future.

On Tuesday, a report in the newspaper Le Parisien alleged that the two Fillon children who had received public funds had subsequently transferred the money into their parents’ personal account — the daughter to pay them back for her wedding expenses, the son to pay them back for rent.

Those revelations followed a report by the Journal du Dimanche newspaper over the weekend that Fillon had asked an undisclosed friend to write a 13,000-euro check ($13,800) last month to pay for the candidate to have two custom suits made by an exclusive Paris tailor.

So what?” Fillon said Monday in an interview about the suits.