In a BVA poll for Orange, Hamon fell 1 percentage point in a week to 12.5 percent, coming fourth in the first round behind far-right leader Marine Le Pen with 26 percent, Macron with 25 and conservative Francois Fillon with 19.5.
Hamon, who unveiled his full manifesto this week, has slid 4.5 points since the beginning of February, and is now just 0.5 points ahead of firebrand leftist Jean-Luc Melenchon.
Both candidates have categorically ruled out pulling out of the race, meaning a divided left is likely to be eliminated from the crucial second-round runoff for the first time since Le Pen's father Jean-Marie's shock second place in 2002.
Fillon has been losing support since becoming embroiled in a scandal over employing his wife as his parliamentary assistant. Macron is seen in the BVA poll making it to the runoff and defeating Le Pen with 62 percent to her 38.