The European Union is on the verge of launching an unprecedented exercise in public consultation.
At stake is the approval of its first constitution by all 25 member states. About 10 countries will hold referendums, with the first one in Spain on Sunday.
Most Spaniards are expected to say "Yes".
Just say Yes: Spain is seen as a safe bet for ratification
But no one knows what will happen if, over the next year or so, one or more countries actually reject the EU constitution.
The problem is that, according to polls, only 11% of EU citizens have actually read the "mother of all EU treaties" or have any idea what it is all about.
First of all it is supposed to simplify all the previous five treaties - but in the English version it still runs to a massive 474 pages.
It streamlines the way decisions are taken - and who does what - in an EU that now includes 25 countries, rather than the original six. And it gives the enlarged EU a higher profile in the world, with its own president and foreign minister.
The constitution also includes a charter of fundamental rights for EU citizens, such as non-discrimination and the right to strike.
It does away with national vetoes in some key areas such as police and judicial co-operation, but it does not bring much change in other sensitive areas, especially taxation, foreign and defence policy - where national governments keep their veto rights.
So overall, what is really in it for the man and woman in the European street - all 450 million of them?
The Vice-President of the European Commission, Margot Wallstroem, is in charge of selling the blueprint to the European public.
"I think one of the more important changes is that it establishes the rights that you have as a citizen of the EU, the fundamental rights that are very, very important," she says.
"So I think that from a symbolic point of view, it's a fundamental change and it establishes the common values for the EU. From a policy content point of view, the changes are minor, they are really not very important.
"But it establishes also the way we should take decisions in the future. With 25 member states in an enlarged EU, how can we be effective, how can we be more open and democratic in the future?"
Referendums are democratic, but notoriously blunt instruments. It is practically impossible to say a simple "Yes" or "No" to complex documents such as an EU treaty.
Kirsty Hughes, an expert on EU matters, thinks that most people will not actually vote on the precise contents of the European constitution.
"I think on the whole we can expect that the vote will be on Europe," she says.
Some may use the vote to protest over regional issues
"It may tend to be more a vote on whether people are in general happy with the EU today rather than the specifics of the constitution. But I think that will vary from country to country.
"In some countries, for instance we saw it with Ireland [a neutral country] when they voted first "No", then "Yes" to the EU's Nice Treaty, they were very bothered with the defence provisions of the treaty."
For the moment, polls show that the most likely country to reject the constitution is the UK.
Other countries where the result is in doubt are Denmark - which in 1992 rejected the Maastricht treaty by a margin of about 2% - and two of the more eurosceptic new member states: the Czech Republic and Poland.
Hostility against the European constitution is also growing in two of the EU's founding members - France and the Netherlands.
"In the UK it would likely be a general negative reaction to the EU as a whole, it wouldn't be to specific provisions in the constitution," says Kirsty Hughes.
"It would reflect the growing scepticism in the UK.
"I think in countries like Poland and the Czech Republic it's perhaps more to do with the fact that they're newcomers to the EU, they've only signed up to one treaty and at least some parts of those countries' publics and politicians ask, 'Why should we have to change already?'"
In the Netherlands, there is public concern over immigration and the integration of the Muslim community.
Similar fears are reflected in France, where many oppose their government's support of Turkey's eventual admission into the EU.
There is also rising social discontent, both over the French government's changes to the 35-hour week and to a proposed European directive that would open up public services to eastern European companies employing cheaper labour.
The French only narrowly approved the Maastricht Treaty (by a 2% margin in 1992), so what if they reject the constitution?
Kirsty Hughes says working out how to resolve such a scenario will not be easy - especially when the constitution is meant to appeal to the public.
"There's never been a situation where the EU has negotiated a treaty and not managed to push it through," she says.
"Will there be a core Europe of a group of countries going ahead?... The EU will still be there, the single market will carry on functioning, but there will be a hiatus for one or two years, with big, big political discussions about what are the options now."
So does the EU have a Plan B?
Margot Wallstroem says not.
"But of course this is a question that many people ask, what would happen? The only thing we can do is to explain what happens procedurally if there is a 'No' in one member state," she says.
"Then of course the European Council would have to look at the reasons why there is a 'No'. It might be because in one small member state actually the voter turnout was so low that it might be worth investing in having another debate and maybe even a new referendum. While if in a big member state there is a clear 'No' on a substantive issue, that changes the whole debate."
In the past, both Ireland and Denmark held two successive referendums to ratify a particular EU treaty. Both passed only after changes to the text of the treaty, answering specific concerns. That will be much harder to do in the constitution, which sets out the fundamental rules of the game.
But for the first time, it allows a country to leave the EU if it so wishes, or indeed be excluded from the bloc if it fails to respect its basic values.
Margot Wallstroem says she does not anticipate either of these provisions being used.
"Today, it's such an integrated co-operation between the member states in the EU that I can hardly imagine how this would actually happen that a member state, and a big member state at that, would say, 'Now we leave this co-operation'."
Difficult to imagine, but not impossible. The UK, already outside the mainstream of European integration - the euro and the Schengen border-free zone - may be the last to hold a referendum.
If, as many think, it ends up rejecting the constitution, after all the other 24 EU members say 'Yes', Kirsty Hughes can only see one possible scenario.
"I think it's going to be very problematic. I haven't talked to anybody who thinks you can ask the British public to vote twice," she says.
"I think that would cause uproar in the British media. But that means you're going to face a very serious political crisis. You'll have a lot of people in the UK, maybe the Conservatives, saying legally we can stay in, legally they can't throw us out. But the political pressure will be to renegotiate, to have some sort of special partnership, for Britain to be like a big Norway.
"And that's going to be a big crisis in Britain as much as in Brussels."
The disadvantage of being like Norway is that a country would have to pay a hefty contribution and follow most of the EU's rules in order to keep up economic and trade links with the bloc, while not actually taking part in decision-making. Whether any of the EU members will want to go that far, is a question no one can answer now.