Sarkozy and his Union for a Popular Movement, or UMP, are being blamed for blurring the lines between the mainstream and extreme right by taking up National Front themes, including the need to preserve France's national identity or try to ensure a low profile for Muslims.
Socialist Prime Minister Jean-Marc Ayrault accuses the conservatives of creating a "strategic alliance" with the National Front.
"There is no alliance," UMP leader Jean-Francois Cope said in an interview Friday in the daily Le Figaro.
"The French must understand that if the left gets all the power Sunday, it's like signing a blank check for five years," Cope said.
The Socialist Party already controls the Senate and regional governments. Adversaries say the majority it expects Sunday would amount to a "Socialist state" in France.
Any candidate who won support of more than 12.5 percent of registered voters in the first round advanced to Sunday's runoff, and many districts have three-way races, including with National Front candidates.
Nadine Morano, a former Sarkozy minister battling for a parliamentary seat in the eastern Moselle region, has publicly reached out to National Front voters "who share our values, my values."
"I don't hear extremist words coming from their mouths," she said this week on TF1, in the company of former Prime Minister Francois Fillon on a campaign outing.
For Le Pen, the isolation wall "has imploded."
The conservatives "have evolved under the pressure of their voters and their base," Le Pen was quoted as saying Wednesday in the online publication Le Telegramme. "A very large majority of UMP voters feel close" to National Front's views.
The anti-Racism group SOS Racism denounces deal-makers as the "candidates of shame."
Says the No. 2 in the Socialist Party, Harlem Désir: "The extreme right is advancing inside UMP like in Swiss cheese."
Cecile Brisson in Paris contributed to this report.