* Conservative candidate says he has most ambitious programme
* Advisers, backers uneasy as financial scandal worsens
* Mass rally planned on Sunday in Paris to support Fillon
* Party committee to meet Monday to discuss situation
By Sophie Louet and John Irish
AUBERVILLIERS, France, March 4 (Reuters) - Embattled French presidential candidate Francois Fillon defended his political plans on Saturday as the only credible future for the country and vowed to stay in the race, a day before a Paris rally called to demonstrate the strength of his support.
Once the frontrunner, conservative Fillon is mired in a scandal over his wife's pay, and his campaign has been in serious trouble since he learned this week that he could be placed under formal investigation for misuse of public funds.
After a string of resignations among advisers and backers, the 63-year old former prime minister is banking on a rally of supporters in Paris on Sunday to show his detractors that he remains their best hope to win the presidency.
"Brick by brick, I have prepared an ambitious programme, the only one in my eyes that can restore France's vitality," he told a rally north of Paris aimed at outlining his vision for the country.
"I am being attacked, but through me what they are trying to attack is the national recovery and a will to change that you all want. Don't abdicate! Don't give up!" he said.
Opinion polls continue to show he would fail to make the second round of the April/May election, however. Instead, centrist Emmanuel Macron is consolidating his position as favourite to win a second-round head-to-head against far-right National Front candidate Marine Le Pen.
The political committee of Fillon's The Republicans party brought forward to Monday a meeting to evaluate the candidate's situation just 50 days from the election's first round, the party said in a statement on Saturday.
Fillon suffered more blows on Friday when his campaign chief Patrick Stefanini and chief spokesman Thierry Solere both quit and the centre-right party UDI withdrew its support.
A social conservative with a deep attachment to his Catholic roots, Fillon has promised radical reforms to the heavily regulated economy, promising to roll back the state and slash government's bloated costs.
A poll published on Friday may also have rattled the Fillon camp. It showed that if he were to step down and be replaced by fellow former prime minister Alain Juppe, then Juppe would make it to the run-off and eliminate Le Pen in the process.
"The pilot is in the cockpit, the door is locked so it's difficult to talk with him, but we don't want the plane to crash," Jean-Baptiste Lemoyne, a senator from Fillon's The Republicains party told LCI television on Saturday.
Bruno Le Maire, Fillon's former adviser on international affairs, who resigned after the candidate revealed he could be placed under formal investigation, said on Saturday that Juppe was the obvious choice.
Fillon has denied any wrongdoing and complained of judicial and media bias that amounted to a "political assassination". His attack on the judiciary in particular has caused unease within his party.
His backers are organising a demonstration of up to 45,000 people on Sunday to show he still carries favour among the grassroots after easily defeating Juppe in a November party primary.
The rally has worried some within right-wing ranks over fears that it will be hijacked by hardline conservative movements.
"It's making me uncomfortable," said Christian Estrosi, the right-wing president of France's southeastern region.
"At a time when we're fighting the National Front, I don't want the ideals carried by my party to be led astray. This rally also seems to want to defy the institutions of our country, and that's not possible." (Additional reporting by Elizabeth Pineau; Editing by Hugh Lawson)