German Chancellor Gerhard Schroeder and French President Jacques Chirac have praised the EU constitution in a joint plea for a French Yes vote next month.
The two leaders are revving up the EU's Franco-German motor
The treaty marks a "major step" in Europe's push for political unity, the two leaders said, while opinion polls suggest French voters will reject it.
The treaty would assert "Europe's weight in the international arena" the leaders said after talks in Paris.
More than 20 opinion polls have pointed to a No in France's 29 May referendum.
France's centre-right government is attempting to convince the 28% of voters who remain undecided to vote Yes.
Rejection by France would scupper the constitution, which needs to be ratified by all 25 EU member states.
"If we vote No we take the responsibility for interrupting 50 years of European construction," Mr Chirac warned at a news conference.
Row over Chinese textiles
He and Mr Schroeder stressed the need for joint EU action to control the flow of Chinese textiles to Europe.
Mr Chirac described the surge in textile imports as "further evidence of our need to unite to take a common stand in the face of globalisation".
Mr Schroeder said the dispute with China highlighted the importance of a Yes vote on the European constitution.
"If a country hit hard were alone, it would be much more difficult to defend itself than if it acted in concert with other countries."
On a recent visit to France, German Foreign Minister Joschka Fischer said the future of Europe depended on the decision of French voters.