Conservative President Nicolas Sarkozy, fighting for his political life after being beaten into second place in Sunday's first round, told a public rally: "I want to talk to the little people, to the foot soldiers, to people in the countryside, to pensioners," saying the National Front leader had drawn a "crisis vote" in "the part of France that is suffering".
"You are feeling afraid," he said. "I have heard you."
Socialist challenger Francois Hollande, who topped Sunday's vote and is favourite to win a May 6 run-off, said in an interview with left-wing daily Liberation: "It's up to me to convince the voters of the National Front."
Mr Hollande put Ms Le Pen's unprecedented 18% score down to despair among "a suffering electorate of office workers, artisans, and blue-collar workers who are really feeling abandoned" as well as farmers struggling to make ends meet.
The two rivals for the second round, which will determine who leads Europe's number two economy, are adopting sharply contrasting tactics to woo swing voters. Mr Sarkozy is playing up divisive issues such as "real work" versus "assistance", while Mr Hollande is projecting himself as a calm, unifying presidential figure.