The first round in the fight for Valls's former seat in Essonne, a working-class constituency near Paris, takes place on Sunday 18 November.
The by-election was caused by his resignation in order to stand for mayor of the city of his birth, Barcelona.
There are 11 candidates but, according to the media, the real contest is between Farida Amrani of the hard-left France Unbowed (LFI) and Valls's former right-hand man Francis Chouat, the 70-year-old mayor of Evry, a town in the constituency.
LFI and Valls have a history of mutual detestation.
As most MPs gave the former Socialist prime minister a standing ovation on his last appearance in the French National Assembly, LFI MPs waved posters declaring "Good riddance".
They accuse him of being a right-winger in left-wing clothing when he was in government and accused his side of cheating their candidate out of victory in the June 2017 election.
Alliance against LFI
Amrani, a 42-year-old civil servant and trade union activist, is standing again in this by-election and her message to voters is to "finish the job".
Like Valls, Chouat was one of the first Socialists to declare their support for Emmanuel Macron's presidential bid and he is supported by the president's Republic on the Move (REM) party, and not by his former party, which is backing Green candidate Eva Sas.
He has also won the support of the five other mayors in the constituency.
They are right-wingers but they are backing the former Socialist in a united front against LFI, which he describes as "populist" and "a danger to democracy".
Mélenchon - asset or liability?
That is a "wretched caste alliance", according to LFI leader Jean-Luc Mélenchon, speaking at a public meeting in support of Amrani three days before the poll.
The candidate's team was not sure whether to invite the former presidential candidate to speak, according to the Mediapart website, because of the bad publicity he received when he confronted police raiding his home in relation to accusations of campaign finance cheating and fake jobs in the European parliament.
His standing in national opinion polls took a hit after that fracas, but, the site says, LFI found he remained as popular as ever in the working-class housing estates that largely voted for him in the 2017 presidential election.
Mélenchon invited Essonne voters to elect the 18th LFI MP and the party has launched an all-out drive, including visits by four other sitting MPs, to win the by-election.
It had hoped to convince other left-wing parties to join it in a united front but failed in that endeavour - not only is Sas standing but there is also a Communist candidate, Michel Nouaille.
Nouaille has promised to support Amrani in the almost inevitable second round but Sas seems unlikely to do so.
Despite that disunity, Chouat has a tough fight on his hands.
LREM's support could be a poisoned chalice, with Macron's support slumped to just 25 percent of the population, according to the latest opinion poll and the Yellow Vests protests against fuel-price rises taking place the day before polling day.
His alliance with right-wingers could also cost him votes in a constituency that is traditionally left-wing.
A challenge for all candidates is the turnout.
At 40 percent, it was low by French standards at the general election and by-elections traditionally see fewer voters going to the polls.
Amrani's camp accuse Chouat of wanting a low turnout, since it is voters from the deprived housing estates where they are strong who are most likely to not make it to the polling stations.
They can only hope that the national media coverage and their own campaigning efforts will reverse that trend.