PARIS — Francois Hollande, a mild-mannered French Socialist, is heading into a presidential runoff election with the upper hand over incumbent Nicolas Sarkozy after winning the most votes in the first round of voting.

However, the nearly one in five voters who chose far right candidate Marine Le Pen may hold the key to victory in the decisive vote on May 6.

Hollande remains favorite to win the runoff after besting Sarkozy in the first round.

If Hollande wins the second round, he would become the first socialist president since 1995. His election could also alter Europe’s political and economic landscape.

Both Hollande and Sarkozy were charging back into campaign mode today after a two-day pause to comply with election regulations that forbid campaigning from midnight Friday until 8 p.m. Sunday.