"It's very much the end the left as a dominant, governing party," said Gérard Grunberg, a leading expert on the history of the French left at Sciences Po in Paris.
This, experts say, is largely because Hamon promises an unrealistically utopian vision of French society unlikely to sway voters from the now-global appeal of populism and its emphasis on national identity and national security.
Having proposed a universal income - initially €750 per person per month - that would cost close to 30 per cent of France's gross domestic product every year, Hamon has consistently polled behind both the race's centrist candidate, Emmanuel Macron, and its leading conservative contender, François Fillon, currently mired in a public spending scandal.
Polls also suggest that Hamon might not even make the second and final round of the vote against Marine Le Pen, the leader of the far-right National Front party, largely defined by xenophobic and Islamophobic rhetoric.
If many voters may have simply voted against Hollande and his legacy, Grunberg said, they ultimately chose a candidate unlikely to appeal to the nation at large - even a nation with as proud a Socialist tradition as France.
"In the end, there is also this evolution in the ideology of the left," he said. "The 'left of the left' is becoming stronger, and its ideology is anti-capitalist and anti-liberal before all. There's a growing distance between the left and social democracy as we know it."
With little chances of a leftist victory in sight, Hollande warned the French public of the dangers behind the alternative.
THE FIVE MAIN CANDIDATES
FRANCOIS FILLON, 62
Fillon won the conservative nomination in November. He's campaigning on promises of drastic free-market reforms, a hard line on immigration and Islam, support for traditional family values and friendlier ties with Russia's Vladimir Putin. Allegations that his wife, Penelope, held a fake but handsomely paid job as a parliamentary aide disrupted Fillon's campaign during the last week. Polls suggest his biggest obstacle to advancing in the general election may be far-right leader Marine Le Pen.
BENOIT HAMON, 49
Comparatively inexperienced, Hamon was chosen as the Socialist nominee on Sunday, defeating former Prime Minister Manuel Valls in a primary runoff. He is a former junior minister and briefly served as education minister under President Francois Hollande. Hamon then rebelled against Hollande's shift towards more business friendly policies and left the government in 2014. His signature proposal is to give a "universal income" of €750 gradually to all adults. The Socialist candidate is now squeezed between far-left and centrist rivals.
MARINE LE PEN, 48
Far-right leader Le Pen, who has strong anti-migrant views, wants to strengthen France's borders and reinstate its national currency, the franc. Since inheriting the leadership of the National Front party from her father, Jean-Marie Le Pen, in 2011, she has ditched its long-standing anti-Semitism to focus on economic protectionism and fears of Islam. The makeover has boosted the party's fortunes among French voters before the northern spring presidential election. Early polls show Le Pen may be among the two top contenders in the first round of the two-part election and advance to the runoff.
EMMANUEL MACRON, 39
Centrist Emmanuel Macron, 39, is campaigning on pro-free market, pro-European views. He suggests loosening some of France' stringent labor rules, especially the 35-hour workweek, to boost hiring. Macron is a former investment banker. He became Hollande's economic adviser at the Elysee Palace in 2012 and two years later, economy minister. He left the government last year after he launched his own political movement, "In Motion" (En Marche). He never has held elected office.
JEAN-LUC MELENCHON, 65
Outspoken Jean-Luc Melenchon, 65, is a former Socialist who left the party in 2008 to create his own far-left movement, the Left Party. Presenting himself as the people's candidate, he is calling for reforms to make the European Union "more democratic" and advocates environment friendly measures. He promises a €1300 minimum wage for employees, up from €1149 now. Melenchon was a candidate in the 2012 presidential race, coming in fourth with 11.1 per cent of the votes in the first round.
-AP