Hollande and Sarkozy traded insults last week in the only televised head-to-head debate of the campaign. Sarkozy labeled Hollande a liar and a "little slanderer," while Hollande accused the president of shirking his responsibilities, cronyism and favoring the privileged over France's poor.
Both candidates reached out to France's undecided voters since the first-round vote on April 22 left them the only two standing.
Centrist Francois Bayrou, who took 9% of the first round vote, delivered a boost to Hollande's campaign Thursday when he said he would vote for the Socialist and urged his supporters to vote according to their conscience.
Sarkozy has been president since 2007.