A student movement has emerged to lead the anti-Chavez campaign, amplifying the condemnation from traditional opposition parties, the Roman Catholic church, rights groups and the defectors who say the reforms are authoritarian.
Chavez, who refuses to scrap term limits for other officials, would have to leave office in 2013 without the law change.
He urged Venezuelans to see vote as a referendum on him.
This year, Chavez has hurt his standing by shutting an opposition TV station and failing to end some food shortages.
Amanda Aguilar, 17, was in line at 5:30 a.m. waiting for a food store to open to buy her single, rationed carton of milk.
The student from Chavez's home-state of Barinas hopes the referendum will force him to change or force him out of office.
"All this lining up ends come December 2," she said.

I can't picture Chavez leaving office peacefully. We can hope, along with those in Venezuela who would like to see him vanish, however I feel he will do anything to retain power.