Former prime minister Manuel Valls endorsed the centrist candidate Emmanuel Macron.
Here are three things that happened in the campaign on Wednesday:
Former prime minister Manuel Valls endorsed the centrist Macron, becoming the most high-profile Socialist to back the 39-year-old ex-banker and former Socialist minister over the party's own nominee.
Valls' support was seen as a mixed blessing for Macron, giving him the backing of a leading politician but weakening his claim to represent a break with the past.
Macron quit the Socialist government last year to form his own movement, En Marche (On the Move), saying he wanted to shake up the political class.
Socialist Party chief Jean-Christophe Cambadelis said he was "sad" he couldn't stop Valls endorsing Macron, but warned against an internal purge, saying it could hurt Socialist candidate Benoit Hamon.
A "Socialist Saint-Barthelemy" would be a bad idea, he said, referring to a 16th-century massacre that bloodied the streets of Paris at the height of the wars of religion.
No one should be "stigmatised, excluded, cut", he said in a letter to Socialist party members.
However, Hamon himself said Socialist defectors had "stabbed him in the back" and that those attracted to this "morbid game" should be punished.
"I'm not afraid of the void," outgoing President Francois Hollande said while wrapping up an Asian tour in Malaysia.
The 62-year-old Socialist leader, who decided in December not to stand for re-election, said he would read and travel a lot and "be fully myself" once freed of his presidential obligations.