The vote is seen as a test of the popularity of President Evo Morales, the leftist populist and the first president in Bolivian history from the country's indigenous ethnic majority.
Morales brought the long-simmering domestic struggle over Bolivia's oil and natural gas reserves to a head in May by nationalizing the country's gas and oil industry, ordering the military to occupy energy facilities.
Morales campaigned this week for voters to give his party - the Movement Toward socialism Party (MAS) - 80 per cent of the seats on a new 255-member constituent assembly so it can rewrite the constitution and prevent any legal challenges to the seizure, among other issues.
But the relatively rich regions around Santa Cruz and Cochabamba, which are seeking more autonomy and control over their energy reserves and other resources, also have a spot on Sunday's ballot - a referendum on whether the country's nine provinces should have more say.
About 500,000 people demanding autonomy for Bolivia's wealthiest state gathered in Santa Cruz at a rally this week to highlight their 'yes campaign' that would squeeze the outflow of resource wealth to the central government and poorer regions.
On Thursday, Morales, 46, dressed in a traditional Indio poncho and a necklace woven of flowers, retaliated with a lively attack on backers of the referendum and defended the constituent assembly in an effort to mobilize voters against apparent citizen apathy. He called for voters to 'restore the inherent social power of all provinces' and end 'colonial' rule by the wealthier states.
'We want affirmation of non-autonomy for the bourgeoisie,' Morales said. 'We want a democratic revolution.'
The short burst of campaigning must by law end Thursday at midnight.
Morales has received backing for his policies from Venezuela's left-wing nationalist President Hugo Chavez, who in May committed his country to buy 100 million dollars of Bolivian government debt and spend more than 1.5 billion dollars in Bolivia on oil exploration and refining, industrial development and health care.
That involvement by Chavez drew fire from Morales' nearest challenger in December's election, centre-right politician Jorge Quiroga, who denounced the moves as interference from outside the country.
Chavez has made Bolivians 'sing hymns of another country,' said Quiroga.
Sunday will mark the fourth time Bolivians have voted in national elections in less than two years. The volatile energy issue brought down previous governments, including that of president Gonzalo Sanchez de Lozada, who was forced to drop participation by Chile in Bolivia's gas industry, and Carlos Mesa, who was the subject of intense protests demanding nationalization.
May's nationalization marked the third time the state took over the energy sector, starting in 1937 and again in 1969.
Bolivia's natural gas and oil sales abroad were 1.3 billion dollars last year, or about half the value of the country's total exports, Bloomberg financial news agency reported.
The constituent assembly is to convene on August 6.
