Neither candidate scored a killer blow in Wednesday night's debate, so the rivals to take on President Nicolas Sarkozy next year sharpened their attacks ahead of Sunday's second-round primary vote.
Martine Aubry, the 61-year-old mayor of the northern city of Lille who came second in the first round of the primary, hurled accusations of being right-wing at frontrunner Francois Hollande.
She said Hollande had used "right-wing terms" during exchanges in the debate over health care and France's 35-hour work week, which Aubry introduced.
"It always disturbs me when a man of the left uses the words of the right," she said on RTL radio, also accusing Hollande of being "vague".
"I think the French understand that I am in the best position to defeat Nicolas Sarkozy in 2012," she added.
Hollande, a 57-year-old former party leader and lawmaker, accused Aubry of making personal attacks and sowing divisions within the Socialists.
"I don't want to be devaluing people, I don't need to denigrate, devalue and denounce," he told Europe 1 radio in response to Aubry's attacks. "I never do anything that can offend and cause divisions in my own camp."
Hollande won the first round of the primary vote on Sunday with 39 percent, and has been endorsed by three of the four defeated candidates, including his former partner, the defeated 2007 presidential candidate Segolene Royal.
Aubry came in second with 31 percent but is expected to pick up many of the votes cast for the third-place challenger Arnaud Montebourg, who won a surprise 17 percent on a platform of protectionism and market regulation.
The run-off takes place at 10,000 polling stations across France on Sunday and stakes are high. Opinion polls suggest either leading Socialist candidate would beat Sarkozy in next April's presidential election.
While Wednesday's debate remained civil it was terse, with Aubry attacking Hollande's lack of ministerial experience and implying that he was not up to the challenge of defeating Sarkozy.
But Hollande kept his cool, defending his record as a campaigner and saying he would be able to represent a new generation and renew French public life.
"Faced with the hard right, faced with a tenacious crisis, we need a strong left... to call the banks to order, to switch to a green economy and get us out of nuclear power," Aubry declared in the opening exchanges.
"I don't want a hard left," retorted Hollande, promising to rally voters. "We're just coming out of five years of a brutal presidency. Should we have a divisive candidacy? I don't want that. We need a solid left."
The primary itself -- the first time a French party has held a US-style open vote to choose a standard bearer -- has boosted the left, mobilising its base, dominating media coverage and drawing 2.7 million to the polls.
Wednesday's debate drew an average of 5.9 million viewers according to state broadcaster France 2.
For it to serve as a springboard, the remaining candidates will have to get to Sunday without publicly demolishing the other, so that the party can unite around whichever wins and begin the campaign against Sarkozy.
The right has been scathing about the poll, attempting to play down the greater than expected turnout, questioning the voting criteria and insisting the debate only underlines the divisions in the left-wing camp.
But most independent observers say the exercise has been a success, so far.
Opinion polls still predict a narrow victory for Hollande, the longstanding frontrunner, but the same pollsters overestimated his support and underplayed that of Aubry and Montebourg before the last vote.