I would like Le Pen's percentage to be really low of course. But Macron was already, I think, a mentally highly unstable person, completely exalted with his own self. Listening to his meetings was just scary, it was like a televangelist show.
So on the back of a huge percentage, all bets are off...

Don't try to second-guess my vote from these lines - I am just pointing out that the question is not who will be president but by which margin.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi

One of Mr. Mélenchon's top aides derided the candidate's critics in a telephone interview Tuesday. "You've got to look at where the criticism is coming from," said Éric Coquerel, a member of the Paris regional council.

"It's coming from those whose policies have favored the development of the National Front, from the Socialist Party," said Mr. Coquerel, referring to the quarrel that divided the French left for five years: the governing Socialists' mild pro-market turn, seen as a betrayal by France's far left.

"We don't want to help Marine Le Pen, but we don't want to endorse Mr. Macron," he said.

"He's the candidate of free trade," Mr. Coquerel said. "He's going to assist in the Uberization of society. Everything we are going to fight against in the coming months. There's no possible rapprochement."

This is a perfectly reasonable stance. Of course Centrists are desperate to sell their horse shoe theories about the Left secretly preferring the fascists.
Especially in France this demand to kiss the centrists ring has nothing at all to do with keeping the fascists out. They want to pull everyone into their TINA non-politics, effectively murdering democracy. After all they don't even care for any of the things the fascists would destroy.
 Our very reasonable centrists are doing their best to have refugees drown in the Mediterranean, build border fences across Europe, support the worst kind of despots around the globe, conspire to provide international companies with veto power over public policies and are now chomping at the bit to hand broad censorship powers over to private companies to prevent "fake news". Sure Le Pen would be worse but they are not offering any principles to adhere to.

Personally I didn't qualify to vote 15 years ago (it's complicated - I was candidate in the 2002 legislatives, but wasn't on the electoral roll), but I handed out Green Party leaflets calling for voting Chirac.

The more salient reference is that most of them voted for Hollande five years ago, for a program that was quite decent on paper (Piketty wrote most of his economic program!), and look at the wasteland he left behind him.

Now, Hollande is the most unpopular president in history, and we are invited - no, commanded - to vote for his spiritual son, his successor in all but name, who will do nothing other than continue and deepen his neo-liberal reforms. So we are doubly cuckolded. Macron will deepen and widen the social chasm that leave an unhealthy plurality of citizens economically disenfranchised, marginalised, precarious, teetering on the edge of poverty or already in it.

These are the Deplorables that vote for Le Pen, and in part, for Mélenchon (and that is his triumph in this election -- a few more weeks of campaign, and he would have won it). According to the polls, about 20% of Mélenchon's electors will vote for Le Pen, 50% for Macron while holding their noses, and the rest will abstain or vote blank. What he recommends will have little effect on that split. And he needs those of his electors who will vote Le Pen next week, and as many of their peers as possible, for the legislatives in June.

More than anything else, what has sunk the French left is over-intellectualizing the vote, and under-emoting. I'm listening to my own emotions, and trying to commune with the herd. I'll vote according to reason, because that's who I am, but with empathy.

_{It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II}

How? Where do I start?

First, people now know that, rather than taking strong consideration of the fact that more left wing than right wing voters voted for him in 2002, Chirac went full throttle, claiming a plebiscite for right-wing policies.

But also, in 2002, at least candidates had been running on their platforms. Macron was running on his person (for very long without a program at all, and even when it came you could see that it was a PR exercise in dropping in key words to give you the impression that he agreed with you, whatever your position).
But worse: the argument paraded throughout the campaign was that you have to vote for me to block the FN, and in the second round it is your duty to vote against them. This essentially turned the system into a first past the post from the get go, whereas in 2002 this was only following an unexpected result in the first round.
First past the post is a terrible system for parliament, let alone for president...

Third, Marine is not Jean-Marie. No, I am not trying to defend her. We must, though, at least acknowledge that they are different people.

Fourth, it could be argued that, economically, Macron is to the right of Chirac, at least on some key considerations that may matter a lot for some voters.

Fifth, in 2002 people had not yet experienced the 2007 crisis and the grinding impact of Germany's pig-headedness (and Macron's desired Europe is very much one where France would simply be in a position to do the same as Germany). Admittedly, France only felt the pinch of the Eurozone crisis as a whole - that is big enough- but for those who care for the plight of Greeks and Spaniards, voting for someone who wants more of it can be very painful.

That's just off the top of my head. I'm sure you can find other reasons why this would be the case.
Again, don't try to read my vote from this. I am simply pointing to a number of reasons why some people could feel different today.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi