Europe could face a "nightmare scenario" between a hard left or hard right French President who wants to sever the country's connection to the Union following the first round of elections on Sunday.
That's the fear of money markets and European leaders ahead of the vote that has four main candidates all within the margin of error.
While 11 politicians remain in the race, polls suggest two centrist candidates; Republican Francois Fillon and Emmanuel Macron of En Marche, will battle far-right Marine Le Pen and hard left Jean-Luc Melenchon to make it into the second round.
Three polls showed Macron with a slight edge on Le Pen at 23-25 per cent against 22-23 per cent. Fillon and Melenchon were tied at around 19 per cent.
Experts predict the National Front leader Le Pen is likely to gain a place leaving a spot open for one of the other three.
If it turns out to be Melenchon, research fellow Matthew Goodwin from UK in a Changing Europe said it will rock the foundations of Europe.
"If Le Pen makes the second round whoever is against her will win," he said.
"I really believe that but the nightmare scenario for Europe is actually if that other person is Melenchon. That's the nightmare position. Not because of the margin ... but because you will have two anti-EU candidates and the left candidate will triumph by rallying an anti Le Pen vote."
"The symbolic blow to the EU of having those two candidates in the final round ....is an interesting dynamic."
Financial markets were nervous in the lead up to the vote with a prospect of a return to the France under Le Pen or ripping up the eurozone treaties under Melenchon.
"If France leaves, the euro is finished," Goodwin said. "Even if [Le Pen] is elected there's going to be a run on the euro."
"I'm happy to be wrong on that but I suspect French savers will rush to the banks to get out their euros as quickly as they can and it won't be long until you have capital controls a la Greece where people are saying actually I can't get more than 60 euros out a day. All of this is running through the minds of voters."
Marine Le Pen is renowned for her far-right views and has vowed to hold a Frexit vote, return to the French Franc and slash immigration to 10,000 a year while regaining control of French borders and cracking down on suspected Islamist terrorists. She hailed the Brexit vote and election of Trump as the start of a new world order and was seen at Trump Tower earlier this year but refused to disclose if she was meeting the new US president.
Melenchon's Unsubmissive party is backed by communists and he has proposed vast economic stimulus, withdrawal from NATO, a constitutional rewrite and advocated an 100 per cent tax on those earning more than EUR360,000 a year in the past. He also wants the debt of poorer eurozone nations written off and has been critical of Germany as an economic imperialist in the past.
Meanwhile Fillon and Macron have much more centrist agendas and pro-European attitudes - something that could be stabilising for Europe in the short term but potentially dangerous long-term if it fails to achieve the real change populist voters are demanding, Goodwin said.
King's College Professor Helen Drake agreed, adding there is also the risk a President without a parliamentary majority becomes a "lame duck" and it could disrupt the fraught Brexit negotiations.
"If Le Pen or Melechon were to win then all bets are off," she said. "A win by either of those candidates we don't know what the effects will be in terms of the markets and so on and so forth but it's not likely to be pretty."
UK in a Changing Europe Director Anand Menon agreed the result could be significant for the fate the UK's Brexit deal, already made complex by the fact the UK will have elections of its own in June.
"If Le Pen won ... my hunch is it makes it a lot harder to see how we get an Article 50 deal at all. The EU would be thrown into such chaos."
"Remember the National Front famously talked about cracking champagne bottles the day after our referendum .... the rhetoric at home in France would be that the EU is being unfair on Britain and it would just throw the negotiations."