Former French President Nicolas Sarkozy has asked for donations in a Facebook appeal to help his cash-strapped conservative UMP party.
The opposition UMP suffered a big blow earlier when election auditors billed it for 11m euros (£9.4m; $14.3m).
It was asked to repay state funds advanced for Mr Sarkozy's 2012 presidential campaign, on the grounds it had breached spending limits.
Mr Sarkozy said the "unprecedented situation" was a threat to pluralism.
He said on Facebook that it "threatens the party that must prepare the really necessary alternative to socialism".
"I have to take on my responsibilities by committing myself to work towards the freedom of democratic expression in our country. I am asking you to help me by mobilising, like me, towards this end."
According to the Constitutional Council, which rules on electoral disputes, the UMP overspent by 2.1%.
Mr Sarkozy resigned from the Council in protest at the move on Thursday.
As a former French president, he automatically became a member of the constitutional body last year.
Hard times for UMP
Party leader Jean-Francois Cope launched a national fundraising campaign after the ruling, which leading centre-right daily Le Figaro described as a "hard blow" for the UMP's finances.
Since its defeat at the presidential and parliamentary elections, the party is believed to have further lost support because of a bitter party leadership battle.
French parties receive grants from the government in proportion to their strength in parliament.
Under electoral law Mr Sarkozy would normally be entitled to have 47.5% of his total campaign spending reimbursed, the AFP news agency reports.
It is the first time a candidate reaching the second round of a presidential election has been refused that reimbursement.
Satirical blogger Nain Portekoi tweeted an image of the UMP's tree logo looking withered in response to the news.
The UMP was allotted 22.5m euros for Mr Sarkozy's unsuccessful contest with Socialist challenger Francois Hollande and was found by the Constitutional Council to have overspent by 466,118 euros.
To ensure an even playing field, France caps election funding, bans large donations and subsidises campaign spending.