The Socialist François Hollande topped the first round vote on 22 April ahead of the rightwing president, Nicolas Sarkozy, creating a dynamic for the left. But Hollande and Sarkozy now face each other in a final run-off on 6 May that is far from clear cut.
The far right dominates headlines after the Front National's Marine Le Pen came third, with the party's highest ever score of 17.9%. Her 6.4 million voters now hold the result in the balance.
Sarkozy has little chance of being re-elected unless he wins over a majority of Le Pen's voters. For the unpopular president, things are extremely difficult but, political experts say, not totally impossible.
Analysts say he needs the support of around 80% of Le Pen voters to win. Polls which vary wildly show on average around half Le Pen voters voting Sarkozy, 20% choosing Hollande and 30% abstaining or spoiling their ballot. Sarkozy's camp says all is to play for, despite recent polls showing Hollande winning.