The conditions demanded by the constitution are not easy to fulfil. Any referendum has to be requested by at least 20% of registered voters - in other words, some 2.4 million people. In August 2003, the opposition produced petitions that were said to contain three million names - but most of them had been gathered earlier in the year, before Mr Chavez had reached his mid-term point, and were ruled inadmissible.
So in November, the opposition tried again, with a four-day signature-gathering marathon that produced a new petition. This time, 3.4 million names were handed in, but the electoral council said only 1.9 million were valid.
In a compromise solution, the council set aside five days at the end of May to allow the owners of disputed signatures to confirm that they did, in fact, back the referendum call. At the end of that verification process, the electoral authorities said the target had been reached and the referendum could take place.