Leaders of several labor and social groups have pledged to take part in the hunger strike.
In January Bolivians approved a new constitution allowing Morales to seek a second five-year term in December's elections, and giving more power to the country's indigenous majority. Morales, Bolivia's first indigenous president, was elected in 2005.
The new constitution came into effect on February 7. By law, the new presidential and parliamentary elections must be held on December 6. Opposition parties believe this will give Morales an unfair advantage in the upcoming elections, and have so far refused to enact the new election law.
Opposition parties delaying the enacting of the new law have demanded an updated voter registry, raised arguments over whether Bolivian expatriates should be able to vote, and contested the number of seats in Congress that should be assigned to indigenous groups.
The first round of debates ended on Thursday after 30 hours of heated arguments, which led to an overall acceptance of new electoral laws. Congress must now agree on all of its 84 points.
Discussions are expected to continue on Friday. Morales is expected to receive the backing of the lower house where his supporters make up the majority, but may fail in the opposition-dominated upper house.
The president of the United Nations General Assembly, Miguel d'Escoto Brockmann, voiced his support for Morales in a statement issued on Thursday through a spokesperson.
"The president of the United Nations General Assembly expresses his solidarity and support to President Evo Morales, who has called a hunger strike [to last] until Congress approves the legislation," he said.
The opposition has called for Morales to end his hunger strike, calling his actions "a presidential diet meant to cover up a lie." The opposition believes the law in its present state is "dark and full of loopholes."
The Argentine daily Clarin has quoted Carlos Dabdub, an opposition leader from the autonomous region of Bolivia's Santa Cruz province, as saying that Morales has taken this stance because he has become "a bit fat and his $1,500 suits don't fit properly."