Fearing a victory by far-right candidate Marine Le Pen in the second round of voting, France's once mighty Socialist Party has thrown its weight behind President Emmanuel Macron.

The Socialist Party unveiled its latest election posters on Friday, bearing the legend: "We all want to say ... but we have to vote Macron," a surprisingly frank summary of the situation in which the Socialists find themselves.

Party head Olivier Faure issued a statement warning of the very real danger that the far-right could come to power in France in the second round run-off. Choosing what the party considers to be the lesser of two evils, Faure urged party supporters to set aside their reservations about the liberal centrist Macron, and to vote for him to ensure Le Pen remained locked out of power.

The Socialist candidate for president, the current mayor of Paris Anne Hidalgo, secured less than 2% of the vote in the first round of the election on Sunday, ending her presidential hopes for good after a lackluster campaign that never failed to get off the ground.