22/04/2017 13:13 BST | Updated 22/04/2017 13:14 BST

A Populist Win For Le Pen Or Mélenchon Would Be A Bigger Crisis For The EU Than Brexit

Eric Gaillard / Reuters

Before Thursday night, the contest to become the next President of the French Fifth Republic was already an unprecedented four-way race. The support in the opinion polls for each of the candidates was within the margin for error, making commentary about who had "inched ahead" or "slipped behind" almost meaningless.

Compounding this uncertainty was an unparalleled number of "don't knows" - excluded from most top-line polling figures - making the result of the first round of the Presidential election this coming Sunday even more unpredictable.

Then, on Thursday evening, as the candidates were participating in the final television debate, Paris suffered another terrorist attack, throwing even more uncertainty and confusion into the mix. What's more, unlike in the UK, where opinion polls can be carried out throughout an election campaign, polling is banned for the final stages of French Presidential elections, making analysis of the effect of the terrorist attack even more difficult.

Sunday's vote will determine which two of the eleven candidates standing to be the next French President will go through to the final round, which will be held on Sunday 7 May. And there is a big chance that the result might well be yet another upset in global politics, as voters once again switch from the established parties and candidates to insurgent alternatives.

If, as is still widely predicted, Marine Le Pen and Emmanuel Macron make it to the second round of the presidential election, neither the Socialists nor the Republicans (or their forebears) will be on the ballot paper for the first time in the history of the Fifth Republic. When traditional parties of government are displaced by new parties or parties previously consigned to the fringes of politics, the mould of politics is truly being broken.

Le Pen looks likely to win a place in the run-off, especially after Thursday night's attack, and polls suggest she will win between 37% and 47% of the vote, depending on who her opponent is.

Compared to her father Jean-Marie Le Pen's 18% in 2002, Ms Le Pen is likely to land a blow for 'right wing populism' even if, as expected, she ultimately fails to win the presidency. While her social platform is clearly of the authoritarian right, Le Pen's economic message of higher government spending is somewhat more left wing, albeit through a nationalist perspective.

And it is not just on the right where the forces of populism are advancing across the Channel. Benoit Hamon, the Socialist candidate from the party of the incumbent president François Hollande, was recently dislodged as the standard bearer of the left by Jean-Luc Mélenchon, a radical hard-left firebrand standing on a platform of a referendum to leave the EU, higher taxes including a top rate income tax of 100%, and an industrial strategy that would make Jeremy Corbyn blush.

Even the pro-European, federalist former banker Emmanuel Macron - the favourite to win the Presidential election - represents his own breakaway movement, En Marche! He has a radical reform agenda that some describe as a form of centrist populism.

While each instance varies in some respect, common themes can be identified across the burgeoning populist support in France and elsewhere, such as the votes for Donald Trump and Brexit. Like them, support for Marine Le Pen is stronger among white voters and those with fewer educational qualifications. And like Trump swing voters and Brexit voters, her support is also correlated with low income levels.

A different story emerges on voters' age, however. While Brexit and Trump support was strongest among over 65s, this age cohort is not Le Pen's strongest. And while younger voters are least likely to back her, many do and her support among the young has attracted much attention.

Digging deeper beyond the headline polling numbers reveals more about what is happening. In my Legatum Institute guide to the French election, James Kanagasooriam and Claudia Chwalisz of Populus found an "enthusiasm gap" between the candidates. They factored in the percentage of that candidate's supporters who say they will not change their mind about who they are voting for.

This analysis suggested Le Pen's vote in the first round would be 29% (5% higher than her headline number) and Fillon's 20.6% (2.1% higher). Crucially, it also reduced Macron's vote by 1.5%. This leaves little more than 1% difference between Macron and Fillon, suggesting a tight race for second place. And following the terrorist attack, there is now a greater chance that voters will be swayed towards Francois FIllon, a former Prime MInister, as a "safe pair of hands".

And if it's a Le Pen-Fillon run-off, there's a good chance that the far right leader will win. Back in 2002, voters on the left supported Jacques Chirac over her father, giving him a crushing 82% victory. But she is seen as a more mainstream politician than her father, and Fillon is seen as a Thatcherite hard-liner, which will put off many voters on the left.

Whatever the result, it is worth following the results, because it could have a big impact on Brexit negotiations. While Le Pen has spoken of wanting to rebuild relations with the UK and Mélenchon says the Brexit voted "must be respected" by organising an exit "without a spirit of vengeance or punishment", victory for one of these strongly eurosceptic candidates would be a bigger crisis for the EU than Brexit was.

A Fillon or Macron victory, however, would represent less of a threat to the status quo in Brussels. While they are both less sympathetic to Britain's withdrawal from the EU, their election would result in greater political stability, making the Brexit negotiations less complex.

But if Le Pen or Mélenchon does win, it will make the Brexit vote and Trump victories of 2016 look almost insubstantial.

Matthew Elliott is a Senior Fellow at the Legatum Institute, and was the CEO of Vote Leave