The pollster changes, but the result stays the same: Jean Luc Mélenchon, the radical left candidate for the French presidency, is surging in the polls. Up to 17 per cent with Ifop, Elabe and Harris, and up to 16 per cent with Opinionway. He is miles ahead of Benoît Hamon, the Socialist candidate, has slumped to 10 to 9 per cent in all the polls.

For Mélenchon, finishing above the Socialist Party – which he left in 2008 after more than three decades as a member and elected representative – would be a victory in of itself, but the best could yet to come. One Kantar poll puts him in third place, ahead of François Fillon, the centre-right’s candidate for the presidency, just six points behind the centrist Emmanuel Macron and the far-right candidate Marine Le Pen.

Crucially, in all of these polls, the gap between Mélenchon and a top two finish – under the rules of France’s electoral system, if no candidate can get more than half the vote in the first round, the top two candidate go through to a run-off – is smaller than what is left of the Socialist Party vote.

Can he do it? Mélenchon’s surges have coincided with the two televised debates, in which he performed well, and was declared the winner of the second, 11-candidate contest. There was to be one more debate before the first round of voting on 23 April – on 20 April. Another strong performance, coupled with a poor performance from either of the candidates he must overhaul could have seen him through to the second round.

But equally, a strong performance from Hamon, an effective debater who was off his game in both debates, could see a reversion to the mean. Hamon is now close to the seven to eight per cent that François Hollande was polling by December 2016, so it may be that he is closer to the Socialist party floor vote than he appears, making Mélenchon’s task harder.

Macron, too, turned in what the pundits generally declared to be a poor performance in the first debate and a slightly improved one in the second, though the polls immediately after declared him the winner of the first and the second-best after Mélenchon in the second. But a stronger performance in the 20 April debate could also have stopped the bleeding and hurt Mélenchon.

In any case, at Mélenchon's urging, that debate has now been cancelled. Mélenchon must now hope that the buzz around his candidacy can carry him over the line, which is a tall order.

That aside, it is still perfectly possible that Mélenchon could beat either Le Pen or Macron into the top two, and face the other in the final round. It's worth noting though, that he enjoyed a similar surge before the 2012 election before falling back. He also underperformed his final poll rating slightly, so there is reason to be bearish about his chances.

What would happen then? There is a maddening dearth of final round match-ups that exclude Le Pen, but many of the wider political forces that would be brought to bear against an anti-system candidate of the far-right would likely be brought to bear against one of the far-left, advantaging Macron in a Macron-Mélenchon contest.

The more difficult question if what happens if Mélenchon faces Le Pen in the final round. According to the polls, he beats her by a greater margin than François Fillon but a smaller one than Emmanuel Macron. (For all that it’s worth, that’s my hunch as to how it would play out too.)

But there’s an important caveat here. The first is that the big variable in the second round is what the defeated candidates do. Do they urge their voters to keep out the candidate of the Vichyite National Front as they did against Marine Le Pen’s father, Jean-Marie Le Pen in 2002? Or do they sit on their hands?

What is undoubtedly true is that France’s large corporations and banks won’t weigh in behind Mélenchon, although the extent to which that is a plus or a minus electorally is up for debate. But it may also be that the centre-right’s politicians sit on their hands or worse, say outright that the two candidates are as bad as each other.

A lot of the bullishness about the obstacles to a Le Pen presidency hold on the idea that the “republican front” against the far-right that the left erected in order to put the centre-right Jacques Chirac back in office. It may yet turn out that the centre-right is less willing to return the favour.