Benoit Hamon won the left-wing primaries on Sunday evening with almost 60 percent of the vote, beating former Prime Minister Manuel Valls and potentially giving a major advantage to independent candidate Emmanuel Macron.
In electing Hamon, left-wing voters have chosen to mark their opposition to the government's liberal line. A former education minister under Manuel Valls in 2014, Hamon was sacked after four months because of disagreements over the economic measures of Valls and President Francois Hollande.
Since then, he has actively opposed the government with other lawmakers in a group called "Frondeurs" (insubordinates). Over the past few years, the group has refused to vote for the budget and filed a motion of censure (which failed by two votes) over a controversial labor law.
With his leftist program, Hamon represents the heart of the socialist ideology in France but also underlines the different views within the party. His main task will be to reunite the wider left divided between the "Frondeurs", the extreme left wing and the more liberal incumbents embodied by Hollande and Valls.
However, Hamon failed to mention this more liberal side in his victory speech Sunday, saying that "from Monday, I will offer Jean-Luc Mélenchon and Yannick Jadot (extreme left-wing and green party candidates respectively) to build a consistent majority". A broad consensus is badly needed for the left to stand a chance in the run-off in the presidential elections in May. The left or center-left has currently four different candidates, with Emmanuel Macron, France's former economy minister, running as independent.
The victory of leftist Hamon gives Macron momentum to gather the liberal establishment and to position himself as the centrist candidate. The coming days will show whether some of Manuel Valls' supporters rally behind Macron instead of Hamon.
Macron could also benefit from the scandal hitting the right-wing candidate Francois Fillon. Prosecutors have opened an inquiry into Fillon over the misuse of public funds after a report that his wife had been given salary as an assistant but had never actually worked in the role. Fillon denies these allegations.
In a poll by research firm Sofres on Sunday - before the Socialist primary results - the data showed that Macron would come third in the presidential election, only one point behind Fillon. Far-right leader Marine Le Pen came first in the poll. In the same poll, Hamon, anticipated as the winner of the primary, would only come in fourth with only 13 percent to 15 percent. This means that Hamon would likely be eliminated in the first round of the presidential election on April 23.