France’s governing Socialist party is set to choose its candidate for the presidential election in April.
Manuel Valls, a former prime minister, and left-wing Benoit Hamon are both contesting the run-off vote on Sunday.
But outgoing President Francois Hollande, a fellow Socialist, is deeply unpopular and may affect the party’s chances in the April election.
Conservative Francois Fillon, right-wing Marine Le Pen, and centrist Emmanuel Macron lead the polls.
Sunday’s Socialist primary has seen Mr Hamon, 49, suddenly surge in popularity with a range of liberal proposals, including a proposal for a universal monthly income for all citizens.
Mr Valls, however, contends that his experience as prime minister between 2014 and 2016, as well as his previous ministerial experience, make him a superior candidate.
Both emerged from a seven-way primary last weekend to enter the run-off race.
But polls suggest neither is likely to retain the presidency for his party.
Mr Hollande, facing extremely low popularity ratings, announced in December that he would not run for re-election.