Aspiring leaders of France’s regions finished cutting deals on Tuesday night in an effort to gain power and also stem the threat of the extreme right during the second round of departmental elections on Sunday.
The first round of voting on 20 June was marked by a record low turnout and abstentions.
“The low number of voters forces us all to be humble,” Castex said. “We will take heed of the consequences of this after the second round of voting.”
Hours after his plea for participation, the intricate plotting ended to head off the political extremes.
In the Ile-de-France region, the three left-wing candidates merged in an attempt to topple the centre-right leader Valérie Pécresse, who came out on top in the first round.
Under the banner, the Gathering for Ecology and Solidarity, Julien Bayou of the EELV ecology party will lead the fight against Pécresse with the backing of the Socialist party’s Audrey Pulvar and Clémentine Autain of the leftist LFI/PCF.
"We're united to win on Sunday," Bayrou tweeted with a picture of Pulvar and Autain.
Further south in the Burgundy-Franche-Comté region, the outgoing Socialist leader Marie-Guite Dufay was given the backing of the ecologist candidate Stéphanie Modde.
Dufay received 26.52 percent of the vote last Sunday while Modde drew in 10.34 percent.
Dufay said Modde's move could help to ward off the threat of Marine Le Pen's far-right RN party whose candidate Julien Odoul registered 23.19 percent.
Denis Thuriot, of Emmanuel Macron's party, LREM, said he would not give his support to Dufay. Gilles Platret of the centre-right LR, also declined.
Even further south in the region of Provence-Alpes-Cote d'Azur - which takes in cities such as Marseille, Aix-en-Provence, Nice and Toulon - the Greens candidate Jean-Laurent Felizia withdrew from a second round run-off vote,
His sacrifice cleared the way for a duel between the LR candidate Renaud Muselier and the RN contender Thierry Mariani.