The slump in the Socialists' share of the vote to just 9.51 percent, following a humiliating 6.36 percent for their candidate, Benoît Hamon, in the presidential first round, is "unprecedented", in the words of national secretary Jean-Christophe Cambadélis.
Even the annus horribilis of 1993 saw 53 Socialist MPs elected, while today the figure is likely to be between 20 and 35.
And even that is deceptive - in percentage terms the Socialists are no longer the biggest force on the left, Jean-Luc Mélenchon's hard-left France Unbowed having won 13.74 percent in the parliamentary first round and 19.2 percent in the presidential vote.
Macron party changes political scene
It was the explosion of President Emmanuel Macron's Republic on the Move (LREM) onto the scene that claimed most Socialist scalps.
Cambadélis himself was squeezed out of the running in his Paris constituency, leaving junior minister for digital technology Mounir Mahjoubi, standing for LREM, to face Sarah Legrain of France Unbowed.
Hamon, too, failed to make it to the deciding round in his constituency in the Yvelines, outside Paris, as was former culture minister Aurélie Filipetti, another figure on the party's left.
Many MPs who had stayed loyal to the government's line also fell victim to the electoral carnage, former interior minister Matthias Fekl and Christian Eckert, who was responsible for the budget, for example.
Not all of them fell to the onward march of the Macron camp.
In Marseille Patrick Menucci lost a hard-fought battle to keep his seat, with France Unbowed leader Mélenchon in the lead against LREM's Corinne Versini.
Macron-backers given stay of execution
Some Socialists have stayed in the running thanks to their support for Macron during the presidential election.
LREM did not stand candidates against former government spokesman Stéphane Le Foll, former labour minister Myriam El Khomri and former health minister Marisol Touraine, for example, and their party affiliation, already ambiguous in their campaign material, will be even more so in the National Assembly should they win reelection.
That is even more the case for former prime minister Manuel Valls.
He quit the Socialist Party before the election but and failed to win LREM's endorsement.
Even so neither party stood against him in his constituency, where he won 25.5 percent and will face France Unbowed's Farida Amrani, who won 17.6 percent, in the final round.
Why the débacle?
The previous government's law banning dual mandates meant that several big hitters chose to remain mayors of major cities rather than stand for parliament again.
But the scale of the débacle indicates that, despite being known in their constituencies, they may not have been reelected anyway.
For the party left, it is the previous government's record in office that has caused the disaster, one of them, François Lamy sarcastically thanking Valls and ex-president François Hollande for the loss of his seat in a tweet.
The party's national executive was to meet on Monday, an event that promised to be less than euphoric.
Meanwhile, Hamon has launched his own movement, without leaving the party, and former justice minister Christiane Taubira and Paris mayor Anne Hidalgo have formed another, all in the spirit of yet another "recomposition" of the left.
Mélenchon fails to repeat presidential success
While France Unbowed and its allies can now claim to have replaced the Socialists as the main force on the French left, they, too, suffered disappointment on Sunday.
They were hit by the low turnout, failing to mobilise all the seven million voters, many of them young, who voted for Mélenchon in the presidential election.
About 50 France Unbowed candidates have made it to the second round and, unless those absentee voters return, the party can expect 11-21 MPs in the next parliament.
Mélenchon came under fire from some quarters when he refused to call for a vote for Macron to stop the far right's Marine Le Pen in the presidential second round.
And, while the Communist Party had, after some hesitation, supported Mélenchon's presidential bid, the hard left went into the parliamentary election divided.
The Communists found France Unbowed's conditions for a united front too strict, so candidates from the two parties opposed each other in the majority of constituencies, leaving 17 Communist Party candidates in the second round.
France Unbowed also turned down offers of deals in some areas with Socialist Party lefts and the Green party, EELV, which says it saw a slight rise in its vote but also defeat for national secretary David Cormand and former housing minister Cécile Duflot.