If Francois Fillon fails to win over France's left, there will be victory for Marine Le Pen

As the French presidential campaign gets underway, with the main parties choosing their candidates ahead of May 2017, the news that former 'Bling Bling' president Nicolas Sarkozy failed to make the run-off in the right's 'Les Republicains' internal elections has caused some consternation. As the most internationally well-known figure, polls predicted a run-off between Alain Juppe, a 71 year old Gaullist figure with a conviction for abuse of public funds and a former prime minister under Chirac, against the flamboyant Sarkozy.

But in a last minute twist, Juppe, long the favourite, was outranked by Francois Fillon, a discreet, former prime minister under Sarkozy in 2007. His popularity has grown over the course of televised debates in which he was considered to be performing well, and as a result Sarkozy was knocked out. Fillon came out on top with 44% of the votes, against Juppe's 28.6%, with the surprise elimination of Sarkozy after he garnered a mere 20.6% of the votes, a result which saw him announce his retirement from politics to focus on his private life.

Juppe is thought to appeal to the party's more centrist supporters, against Francois Fillon, a traditionalist who has presented himself as a more reserved and perhaps less ostentatious alternative, while towing very similar lines – a sort of 'Sarkozy light', if you will.

Fillon was the dark horse of the race. Juppe underestimated Fillon and had focused primarily on distinguishing himself from Sarkozy. But as results from Brexit to the US elections have shown, polls can't always to be trusted and the understated Fillon appears to have struck a chord within the party. A Catholic father of five, Fillon has received the blessing of Sarkozy who called on his supporters to back the former PM.

One opinion poll places Fillon well ahead of Juppe in the next round, suggesting the final run-off could be between Fillon and Le Pen, in the knowledge the Socialist party has yet to provide a candidate able to rival either of them.

Fillon has already come under attack for representing an apparent 'Thatcherisation' of the right, an unflattering comparison in the French context to a woman seen as responsible for bringing trade unions, a powerful political force in France, to their knees.

Perhaps in a political olive branch to socialist voters – unlikely to see their political colours represented in the final round of the elections – Fillon recently announced that in the case of a Socialist/National Front run-off, he would pick the former. That is certainly something he is hoping will be reciprocated by leftist voters if he does make the presidential contest. Though many will be skeptical given that the UMP, the antecedent party to 'Les Republicains', had previously called for a blank vote in such circumstances and Fillon himself had towed the line, describing his position as 'neither-neither' (neither the Socialists, not the National Front).

For their part, the National Front has joined a chorus of voices already picking holes in Fillon's economic plans and relishing the fact the debates in the primaries echoed many of the FN's priorities – national identity, France's role in Europe, immigration and extremism.

A central fear for many in France has been that the disunity within the right, which has been plagued by internal squabbling, and perceived absence of political leadership from the left, where current President Francois Hollande polled almost 90% disapproval over the summer, would leave leftist voters with nowhere to turn. This would leave a possible run-off between the right and the far-right on traditionally far-right themes, which could not only embolden, but possibly hand Le Pen victory. If only through large scale abstention. Over 20% of voters abstained in the first round of the 2012 presidential election which presented a far less polarised political scene.

Currently still under a state of emergency, facing regular threats from terrorism, not least the uncovering of a foiled attack today, the question of national security will continue to dominate the political stakes. But beyond that, serious questions over the nature of some of France's cornerstone institutions are being posed – Fillon wants to cut public spending by €100bn, a figure not dissimilar, if slightly less vague than the promise of his now main rival on the right Alain Juppe.

Both want to do away with the 35h working week, with Fillon pushing for the European union maximum of 48hours, a proposal likely to deeply anger worker unions – who protested on and off for months over changes to employment laws passed in August this year – seen largely as more favourable to companies and as prejudicial to worker rights.

Fillon wants a referendum on quotas for immigration, he wants to ban the burkini (yes, actually!) and he wants to harden the naturalisation process. Such promises will speak to voters vacillating between the right and far-right, the sort of terrain Sarkozy was keen to try and recoup votes from himself, but it's also a dangerous tactic. Some analysts suggest it only lends further credence to Le Pen's agenda by allowing her party to dictate the terms of the presidential debate.

What's more, Fillon's traditional conservatism will likely alienate many on the left who would otherwise choose to vote for Les Republicains to keep the National Front out. His desire to revisit same-sex couple adoption, a rapprochement with Russia over Syria, and cuts to the numbers of civil servants may rule him out completely.

The danger is that between the left's vacuum and the right's shift to right, Le Pen may find the petri-dish ripe for a worrying experiment in French politics.

Myriam Francois is a journalist and academic (Research Associate, SOAS) with a focus on France, Islam and the Middle East.