Former French prime minister Manuel Valls faces an outside chance of failing to make it to the Socialist party runoff in party primaries ahead of the country's Presidential elections.
Candidates who are to the left of Mr Valls, such as former education minister Benoit Hamon and ex-economy minister Arnaud Montebourg, are nipping at his heels in the polls as the pro-business Socialist has struggled to defend his government's record.
Seven candidates from the Socialists and their allies are taking part in the first round of the primary, with polls closing at 6pm GMT. A runoff will then be held on 29 Jan to pick a candidate for the two-round presidential election on 23 April and 7 May.
Regardless of who wins the primary, polls were indicating the Socialist candidate had little chance of making it into the runnoff in the presidential election.
One of the dominant forces of French politics for decades, support for the Socialist party has evaporated during Francois Hollande's presidency as he struggled to turn the economy around and alienated left-wing voters with his economic policies.
The Socialists' choice of presidential candidate will be key for the chances of popular independent Emmanuel Macron, who is attracting middle-ground voters who Mr Valls also appeals to.
Polls indicated conservative former prime minister Francois Fillon was most likely to emerge as the winner of the presidential election in a contest against far-right National Front leader Marine Le Pen.
Mr Fillon's programme included cutting business taxes, relaxing labour laws and scrapping the 35-hour working week in an attempt to boost growth, while also cutting 500,000 public sector jobs as part of a drive to shrink the state sector.
But Mr Macron, a youthful and charismatic one-time investment banker, was gaining ground and could make it into a presidential runoff against Mr Fillon if a leftwinger like Mr Montebourg or Mr Hamon won the Socialist nomination.
A poll last week saw Mr Valls, who stepped down from government last month, coming out on top in both rounds of the primary vote with 37 percent in the first round.
However, his lead narrowed after Mr Hamon made a stronger impression in a series of televised debates, with a proposal for monthly income support payments for all.
Mr Hamon and Mr Montebourg were kicked out of the Socialist government led by Mr Valls in 2014 for criticising its economic policies, which they criticised as too favourable to business.