(CNN) On a night marred by insults, one stood out. "France will be led by a woman, either me or Mrs Merkel," Marine Le Pen said of her opponent, Emmanuel Macron, during a two-hour grudge match on French television as the presidential campaign entered its closing days.

It was a deft jibe, emasculating Macron, and at the same time playing into her supporters' fears of a France crushed by a German-dominated Europe.

It would have resonated in Brussels, too, where the prospect of a Le Pen presidency brings eurocrats out in cold sweats. During the campaign, Le Pen has railed against an imperious EU, suggested France would leave the euro, and promised to hold a referendum on France's continued membership of the bloc.

Marine Le Pen and Emmanuel Macron go head-to-head on French TV.

Such an outcome on Sunday would send shockwaves through the EU, already battling to maintain a united front against a belligerent Britain even before formal Brexit talks have begun.

The fear in Brussels is that President Le Pen, emboldened by Theresa May's hardline Brexit in the UK, will make plans for Frexit as soon as she is able. But unlike Brexit, which involves the departure of already semi-detached nation outside of the border-free Schengen area and the euro, the departure of France, one of the political and economic powerhouses of the EU, would surely spell the end of the bloc.

But even a Macron victory would have ramifications far beyond France's borders.

Europe and Macron

On the face of it, a Macron presidency should bring sighs of relief in Brussels. Macron, a former economy minister under Francois Hollande, is an outward-looking, economically liberal veteran of the Brussels ministerial meeting circuit.

But Macron, who was avowedly pro-EU during the earlier stages of the campaign, shifted to a more Eurosceptic position after the first round, which he won with 24% of the vote. The centrist candidate has had to temper his pro-Europeanism to appeal to voters beyond his support base, including those who backed the far-left anti-EU Jean-Luc Melenchon.

In order to secure a convincing win against Le Pen he also needs to reach out to disaffected voters. In the same way that Americans backed Donald Trump, there is a deep desire for change in France. Yet many voters are thinking of abstaining this Sunday because they feel unable to choose between the far-right and the establishment center.

Emmanuel Macron has hardened his stance on the EU.

As a result, Macron gave an interview on Monday calling for reform of the EU, warning that it would be a "betrayal" if the bloc continued as it was and that a failure to reform would hasten a "Frexit".

Of course, it helps Macron to appear more Eurosceptic as he goes up against Le Pen

But this is more than just an empty threat - it is likely that once established in the Elysee, Macron will go ahead with pushing for reform.

Europe and Le Pen

A victory for Le Pen on Sunday would by no means guarantee France's departure from the European Union. For one thing, she has stepped back from a hardline position as she has sought to extend support from beyond her base.

In any case, membership of the EU is embedded in the French constitution and Frexit would need the approval of the French national assembly, which would almost certainly block the move.

JUST WATCHED Why these French voters support Le Pen Replay More Videos ... MUST WATCH Why these French voters support Le Pen 00:52

But a Le Pen presidency would result in a hardline Euroskeptic Elysee squaring up against Brussels, making trouble for the future stability of the bloc.

Even if she is unable to implement Frexit, Le Pen could call a referendum on the euro -- which she has described as a "weapon in the hands of the European Central Bank" - and press for France to withdraw from the border-free Schengen area.

After the spate of terrorist attacks in Europe, several countries including France imposed border controls and under the terms of the Schengen Agreement, these can be extended to two years in exceptional circumstances -- a move President Le Pen would likely instigate.

EU debate at forefront of campaign

Axelle Lemaire, a French Socialist MP and minister in the current government, said the election had given voters the opportunity for a "real debate" about the future of the EU and France's place inside it. She said: "Between the two rounds each candidate tries to attract the largest majority of voters possible, and so it is not surprising to hear Marine Le Pen talk about how there could be 'co-existence' between the two currencies.":

"Emmanuel Macron is becoming more vocal in his criticism of the EU. He is still a strong supporter of the European project and its ambition but he has been critical of its transparency, democracy and efficiency and whether it is creating jobs and prosperity."

Lemaire said that even if Macron wins on Sunday he would still be keen to have a reformed EU.

"I think he is sincere in his request for reform. It is not possible to accept the situation as it is, and people don't accept it as it is. So this means dealing with Germany, especially if it is run again by Angela Merkel, by saying 'let's try to re-instill ambition in the European project'. I think he will carry out a reform agenda."

Whoever wins, either Le Pen or Macron as president could use the Brexit process to shoe-horn changes to its relationship with the EU, by tacking on demands during the EU27 negotiations. Without a doubt, a Le Pen victory poses the greater threat to the future existence of the EU. Yet even if Macron triumphs, it is clear that the EU will not be able to dodge significant reform for much longer.