Emmanuel Macron has accused election rival Marine Le Pen of hatching a secret plan to drag France out of the EU.
The French President has cast this months’ showdown as a Brexit-style referendum on his country’s membership.
His remarks come with polls showing a run-off contest between the pair is much closer than five years ago.
During a campaign visit, Ms Le Pen refused to rule out holding a public vote on France leaving the bloc.
She said: “The French can reassess their presence on whichever international organisations they choose.
“In the case of a referendum that is against the fundamental interests of the country, the President and the National Assembly can oppose it.
“But that is not the case with an exit from the EU.”
The far-right candidate has previously campaigned for a “Frexit” and said France should bin the euro.
But she has dropped those policies during this campaign to present a softer image to French voters.
Instead, she has pledged to work with other eurosceptics to change the project from within.
Mrs Le Pen wants to rip up the EU’s border-free Schengen zone and reintroduce border checks.
She has also vowed to pull out of the bloc’s electricity market and remove agriculture from future trade deals.
Critics say her pledges would set France on a collision course with Brussels; they amount to Frexit by stealth.
Mr Macron claims that “she wants to leave but does not dare say so” and accused her of “talking rubbish” about the EU.
At a rally in Strasbourg, he said: “She says that she wants an alliance of nation-states.
“But she is going to find herself in a corner, and she will try to come up with an alliance with her friends.
“The EU has changed the life of this country. This election is a referendum on Europe.”
The bloc’s former Brexit negotiator, Michel Barnier, has previously warned France could follow Britain out of the bloc.