The FN saw record gains in the first round of regional polls Sunday, topping the list in at least six of 13 regions, with leader Le Pen and her niece Marion Marechal-Le Pen breaking the symbolic 40-percent mark in their respective areas.
But the Socialists later announced they would pull candidates out of the race in the two regions where the women were standing, calling on their voters to back Sarkozy's centre-right Republicans and keep out the FN.
On Monday morning the party also announced it would withdraw it's candidate in a third region - Alsace-Lorraine-Champagne-Ardennes, where National Front candidate Florian Philippot topped the polls after the first round.
However it then emerged that the Socialist party candidate who heads the list has refused to step down.
Socialist Party first secretary Jean-Christophe Cambadelis called for a "grand coalition" as he announced candidates for the
Nord-Pas-de-Calais-Picardie and Provence-Alpes-Cote d'Azur regions would withdraw.
"We went as far as we could," a party leader told AFP, speaking on condition of anonymity.
Marine Le Pen has however blasted the Socialist party's tactic as "anti-democratic".
But former conservative president Nicolas Sarkozy, head of The Republicans, said again that his party would not drop out to help Socialist candidates beat the FN in close races.
"We must hear and understand the profound exasperation of the French people," he said.
Christopher Castaner, the Socialist who was forced to step down in Provence-Alpes-Cote d'Azur in order to dent the younger Le Pen's chances, said he made the move "with a heavy heart".
"The FN's score is a threat to French values," he said.
Any party which secures 10 percent backing in the first round has the right to present candidates in the second round, due next Sunday.