Rebel Socialist Benoît Hamon, who resigned as education minister in protest at what he saw as the government's rightward drift, has made the basic income proposal his key policy and Jean-Luc Bennahmias, the leader of the small Democratic and Ecologist Union, also backs the call.
And the question provided the only real controversy in the first of three televised debates ahead of the primaries to be held on 22 and 29 January.
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Former prime minister Manuel Valls dismissed it as contrary to his vision of a "society of work", Euro-MP and former education minister Vincent Peillon called it a nice idea but too expensive, while François de Rugy of the Left Radicals complained that it was distracting from the debate on how to create jobs.
Peillon claimed that a basic income would cost 400 billion euros a year, while Hamon and Bennahmias put the cost at 300 billion euros and insisted that it is not an "incitement to laziness".
Reluctance to defend Hollande's record
With the latest opinion polls showing whoever wins the primary in a disastrous fifth position in April's first round of the actual election, most candidates did not seem keen to comment on the record of President François Hollande's government, which is held responsible for the poor showing.
While Valls said he was proud of it, former economy minister Arnaud Montebourg judged it "hard to defend", Hamon said it left a feeling of something unfinished, while Peillon said it had been widely misunderstood.
"Hollande's babies reject the inheritance," right-wing opposition MP Eric Ciotti commented on Friday, claiming that the Socialist had "failed in practically everything".
Hollande has decided not to stand again.
Macron, Mélenchon overtake Socialists
Some 3.8 million people watched the debate, a lower audience than that for the mainstream right's debates, which led to former prime minister François Fillon being picked in primaries in December.
Montebourg was judged the most convincing in Thursday evening's debate, according to a poll of viewers carried out for BFM-TV.
He won 29 percent approval rating, followed by Valls at 26 percent and Hamon at 20 percent.
Among left-wing voters Valls won 28 percent, Hamon 27 percent and Montebourg 23 percent.
Although the Socialists hoped to bring all the left into the primaries, former economy minister Emmanuel Macron has set up his own party and is standing under its banner, while Jean-Luc Mélenchon is standing for the hard left with reluctant support from the Communist Party.
Both Macron and Mélenchon have risen in the polls recently, both doing better than any Socialist candidate, but all still show Fillon facing National Front candidate Marine Le Pen in the deciding round.