The French referendum is seen as the proposed constitution's first big test. France is the first country to put the issue of ratification to a binding referendum. That means that if a majority of French voters say 'no,' French officials will be obliged to reject the document.
The constitution must be ratified by all 25 EU member states before it can take effect. So far, nine countries have approved the charter -- mostly through votes within national parliaments.
EU officials have said there are no clear arrangements for what to do if the document fails to win the support of voters in one or more of its member states.
But correspondents and political analysts say a "no" vote in France is likely to provoke a prolonged bout of introspection as European leaders try to figure out what went wrong and whether the treaty can be salvaged, either in full or in part.
French President Jacques Chirac has thrown his support behind the proposed constitution. He says it will give Europe a bigger voice in world affairs and will make EU decision-making easier following its last enlargement a year ago to 25 member states.
But with one in 10 French workers unemployed, the electorate appeared to be in a rebellious mood leading up to the vote. The last 15 opinion surveys taken before campaigning closed yesterday evening all predicted victory for those who oppose the document. Chirac's opponents say they sense a chance to humiliate the French leader ahead of presidential elections in 2007.
That leaves other European officials who support the document nervously awaiting the results that will become available after the last polling stations close in Paris and Lyon tonight.
The repercussions of the French vote are expected to spread across the rest of Europe. Dutch voters are schedule to go to the polls on Wednesday in a nonbinding referendum. Correspondents say that if the French vote "no," the Dutch are likely to do the same.
And such a double blow could prove fatal for the constitution, which was designed as the next big step in the historic project of bringing Europeans together.
Still, Dutch Foreign Minister Ben Bot was telling reporters today that he is confident of a "yes" vote in both France and the Netherlands:
"I see that in the last few days, the 'no' vote is losing ground and that the 'yes' vote is gaining considerable ground compared to what it was a few days ago," Bot said. "Even if it were to be a 'no' in France, I'm absolutely sure that the Dutch will not be influenced by the French vote."
The treaty always has been considered as something difficult for its supporters to sell to voters. In addition to 448 clauses, there are countless protocols and annexes that are laden with legal language.
It would create the post of a EU foreign minister -- no small advance for a bloc that proved incapable of working together over the Iraq war or preventing the Balkan conflicts of the 1990s.
It is also designed to make collective decision-making and action easier. With EU powers France and Germany experiencing double-digit unemployment and sputtering economies, treaty supporters say they hope the document will give the bloc a badly needed economic boost.