* Melenchon vote key to the second round runoff on April 24
* Macron takes green turn to convince left-wing
* Le Pen denies she is climate sceptic (Recasts with no voting instructions, adds Le Pen, Greens)
By John Irish
PARIS, April 17 (Reuters) - The party of far-left candidate Jean-Luc Melenchon gave no voting instructions for the French presidential runoff after its internal consultations showed most people would abstain or vote blank on April 24, adding to the uncertainty over the outcome.
President Emmanuel Macron and his far-right challenger Marine Le Pen are looking to attract voters who chose Melenchon, after he came third in the first round on April 10 with about 22% of the vote.
With the electorate fragmented and undecided, the election will likely be won by the candidate who can reach beyond his or her camp to convince voters that the other option would be far worse.
After the first round, Melenchon called on his supporters not to vote Le Pen, but he stopped short of advocating Macron and said his party would hold a public consultation to help guide the millions who backed him.
According to results published on Sunday, about 215,000 party sympathizers who participated in that, more than 66% said they would abstain, leave their ballot paper blank or spoil it. Just over 33% said they would vote for Macron. The option of voting for Le Pen was not given to respondents.
"The results are not an instruction to vote for anybody... everyone will conclude from this and vote as they see fit," Melenchon's campaign team wrote on its website.
Pollsters estimate the overall abstention rate for next Sunday's election at about 30%, similar to the first round.
It is not clear what a high abstention rate either overall or among Melenchon voters would mean for either candidate. Both Macron and Le Pen were able to mobilise their core support in the first round, but are struggling to appeal beyond their own camps.
The vote next Sunday is a repeat of the 2017 match-up in the second round. Then, Macron, a pro-European Union centrist, easily beat Le Pen when voters rallied behind him to keep her party out of power.
This time round he is seen as facing a much tougher challenge, although the latest opinion polls give him a nine-10 point lead over Le Pen.
An IPSOS-Sopra-Steria opinion poll on Saturday showed that some 33% of Melenchon voters would back Macron, with 16% supporting Le Pen on April 24. But 51% of people were undecided.
WOOING MELENCHON VOTERS
For decades, a "republican front" of voters of all stripes rallying behind a mainstream candidate has helped to keep the far right out of power.