Alain Juppé would win the first round of the French presidential election should he replace François Fillon, the current Right-wing candidate facing corruption charges and who suffered fresh defections on Friday.
In major blow to Mr Fillon, who has refused to step down despite mounting criticism, the centrist UDI party said it was withdrawing support for him because it views him as "a danger to France."
Mr Fillon called on his supporters on Friday to "resist" and to show their backing for his campaign by turning up in large numbers to a rally in Paris on Sunday.
"Don't let anybody steal this choice from you, I ask you to resist," Fillon said in a video message posted on his Twitter account, calling for a large turnout on Sunday.
Earlier Thierry Solere, Mr Fillon's campaign spokesman, resigned in the latest of a series of defections related to pending charges against Mr Fillon that he used parliament funds to pay his British wife for an alleged fake job.
Mr Solere was the respected organiser of November primaries of Mr Fillon's Les Républicains party, in which he beat Mr Juppé on a more radical economic and socially conservative programme.
Mr Fillon's campaign treasurer and three legislators from his Republicans party announced on Thursday they were quitting, amid growing pressure for him to step down.
Dominique Bussereau, another member of his inner circle, also threw in the towel on Friday.
Mr Fillon has denied wrongdoing and insisted at a rally on Thursday night that he would continue as candidate, saying: “The French people back me.”
But an Odoxa poll suggested that 70 per cent of the French think he should withdraw.
Meanwhile, Mr Juppé, a former prime minister, would come first in round one of the presidential elections on April 23 if he ran, an Odoxa survey suggested.
Emmanuel Macron, the independent ex-economy minister, would come second and Front National candidate Marine Le Pen would be eliminated from the race, the poll suggested.
Je me mets en retrait de la campagne de @FrancoisFillon et démissionne de toutes mes fonctions dans son équipe.— Dominique Bussereau (@Dbussereau) March 3, 2017
After previously swearing he had no interest in being a replacement candidate, Mr Juppé let it be known on Friday that he would "not shirk his responsibilities" should Mr Fillon withdraw.
However, his camp insisted that "all the conditions" would have to be met, namely official backing from supporters of Mr Fillon and ex-president Nicolas Sarkozy, who is reportedly against a Juppé bid.
Several MPs close to Mr Sarkozy withdrew support on Friday, asking Mr Fillon to fall on his sword for the good of the mainstream Right.
Nadine Morano, a former minister said Mr Fillon he was in a “dead end”, his electoral chances slipping away, and he risked bringing “catastrophe" his political camp.
She also urged Mr Fillon's remaining supporters not to go ahead with a planned open-air rally in Paris' Trocadero on Sunday.
Hardline Catholic movement Sens Commun, which still backs Mr Fillon, could reportedly draw in "tens of thousands" of its supporters. In a statement, it urged them to turn up to protest against a "democratic hold up".
A counter demonstration against Mr Fillon's claims that French judges are seeking his "political assassination" has also been called.
Ms Morano warned that the pro-Fillon rally was "irresponsible in the current context of the state of emergency."
"We don't know what can happen," she said.
According to Libération newspaper, which has a page keeping tabs on Fillon defections, almost 70 Right-wingers have dropped their candidate this week.
Mr Fillon initially said he would step down if charged, but decided to maintain his candidacy even though he's been summoned to face charges on March 15.
Mr Fillon denies that he arranged jobs for family members as parliamentary assistants, including his British wife Penelope, who allegedly did no actual work.
There is growing disquiet over the turbulent and unpredictable nature of the French electoral campaign, in particular given the strength of support for the far-Right.
Gérald Darmanin, Right-wing vice president of the northern Hauts-de-France region, said: "Until now, it was the Socialists who were pushing up the Front National score. Now it's us. I'm ashamed of my Right."