France election: Decisive hours ahead for Fillon
Leading members of France's centre-right Republicans are urging their candidate, Francois Fillon, to take part in a unity summit to help resolve the party's crisis.
Mr Fillon has denied allegations that members of his family were paid taxpayers' money for fictitious jobs.
But he has lost support within the party and in opinion polls ahead of the first round of the election next month.
Ex-President Nicolas Sarkozy has called for an emergency meeting.
He says Mr Fillon should meet him and Alain Juppe, seen as the most likely replacement if he withdraws. Mr Juppe is due to make a statement at 10:30 (09:30 GMT).
'Disunity playing into far-right hands'
Until now Mr Juppe has insisted he will not be the party's Plan B, but polls suggest he would have a far greater chance than Mr Fillon of reaching the final round of the presidential vote in May.
Key members of the Fillon campaign team have abandoned him and several leading Republicans have wavered in their support.
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Mr Sarkozy warned that "our disunity will play into the hands of the far right". National Front leader Marine Le Pen is well ahead of Mr Fillon in the polls.
Leading Republican Valerie Pecresse threw her weight behind the idea of a three-way summit, speaking of the urgent need to restore unity. Senior party figures were already due to meet later on Monday.
Mr Fillon told French TV on Sunday evening that "no-one today can prevent me being a candidate". But pressure on him is mounting and the battle between Mr Fillon and his party may be entering the end game, says the BBC's Lucy Williamson in Paris.
At a mass rally in Paris on Sunday, a defiant Mr Fillon told tens of thousands of supporters that he would fight on.
He rejected the idea of being replaced by Mr Juppe, his rival in the primaries last year.
"If they had wanted Alain Juppe's project, then they would have voted for Alain Juppe in that election," Mr Fillon said.
He said he would be exonerated when an impending criminal investigation got under way, and it would be the turn of his accusers to feel ashamed.
For weeks he has fought allegations that his wife, Penelope, was paid for a number of years for work that she did not do as his parliamentary assistant.
However Mrs Fillon, who insists she did work for her husband, told French magazine Journal du Dimanche on Saturday that "everything was legal and declared".
Also under scrutiny are claims that two of the children, Marie and Charles, were paid by their father's office for legal work though they had not yet qualified as lawyers.