French parties are making their final appeals to voters ahead of Sunday's referendum on the EU constitution.
The "No" camp has united disparate aggrieved groups
Leaders from Germany and Spain are due to attend French Socialist Party rallies calling for a "Yes" vote.
But leading Socialist dissidents will join Communists and union leaders in Paris to urge people to vote "No".
Germany's parliament completed the ratification of the charter on Friday, with an upper house vote designed to boost France's struggling "Yes" camp.
The latest French opinion poll, by TNS Sofres-Unilog, suggest 54% of people intend to vote "No", with "Yes" on 46%.
Both the governing centre-right UMP party and the opposition Socialists are urging voters to support the proposed constitution.
On Friday the head of the Socialist party Francois Hollande will be joined by Spanish Prime Minister Jose Luis Rodriguez Zapatero at a rally in the northern city of Lille.
And German Chancellor Gerhard Schroeder is due to take part in a similar meeting in the southern city of Toulouse.
The BBC's Caroline Wyatt in Paris says politicians are far from certain that they will be able to shift public opinion on a vote which has divided France.
Leading Socialist dissidents, including former Prime Minister Laurent Fabius and MP Jean-Luc Melenchon, have urged voters to turn out in force in Sunday's referendum.
Mr Melenchon will join leaders from various left-wing parties and trade unions in a "No" rally in Paris.
Supporters of the far-right National Front - which also rejects the proposed constitution - are also due to gather in the capital.
On Thursday President Jacques Chirac went on TV to call on the French to support the proposed constitution.
He said the vote was vital for the "future of France and the future of Europe".
The French president urged voters not to use the referendum as a vote against the centre-right government.
He tried to calm fears of an attack on France's social model and gave an assurance that the people would be consulted in a referendum over whether to allow Turkey entry to the EU.
The BBC's William Horsley in Paris says the campaign has stirred a lively debate in France about the EU's economic and foreign policies.
But many French voters also say their choice will also be influenced by their views on the record of President Chirac and his government, he says.
The campaign has been overshadowed by a gloomy mood brought on by years of faltering growth and unemployment of around 10%.
4 March: Voting date named
14 April: Chirac TV debate
3 May: 1st Chirac TV address
26 May: 2nd Chirac TV address
Thirteen successive polls in the last fortnight have shown a lead for the "No" camp.
Treaty rejectionists are also ahead in the Netherlands, which will hold its referendum on Wednesday.
The EU constitution has to be ratified by all 25 member states to become law.
France is the second EU member to hold a referendum on the document. Spain's referendum resulted in a "Yes" vote, which was then ratified by both houses of parliament.
On Friday the upper house of Germany's parliament approved the charter by an overwhelming majority. The lower house has already held a similar vote.
The treaty aims to streamline decision-making in the enlarged EU of 25 nations.