LONDON — What is a constitution? The question may seem to be a rarefied and abstruse one for lawyers and academics, but just at the moment it lies at the very heart of British politics and strategy.

The argument is about the character and purpose of the so-called Lisbon Treaty, now before Parliament at Westminster, which restructures the European Union and changes quite fundamentally the relationship between the member states and the central EU institutions .

The new treaty looks suspiciously like the last one, which openly called itself a constitution for Europe, and which was soundly rejected by French and Dutch referendums two years ago. The British, too, were offered a referendum on this constitution, but after the French and Dutch votes it never took place.