LA PAZ - President Evo Morales went on a hunger strike on Thursday to demand Bolivia's Congress pass an electoral law that could make it easier for him to win control of the legislature in December's general election.

Tensions flared last week when members of the ruling Movement Toward Socialism, or MAS, party clashed with the opposition over the bill, which could help the leftist president by assigning more seats to poorer, rural areas.

Allies of Morales control the lower house in the Andean country, but the opposition holds a majority in the Senate, allowing his opponents to block the election reform proposal.

Faced with the negligence of a bunch of neoliberal lawmakers, we have no choice but to take this step (hunger strike) ... they don't want to pass a law that guarantees the implementation of the constitution, Morales told reporters at the presidential palace in La Paz.

Lawmakers exchanged insults during a heated session on Thursday evening as they debated the bill. Opposition delegates called the Morales government totalitarian.

A new constitution designed to give more power and rights to the country's indigenous majority was approved by more than 60 percent of voters in January. Morales is Bolivia's first indigenous president.

The constitution calls for Congress to approve an electoral law ratifying December 6 as the date for a general election.

The opposition has rejected the bill because it gives 14 seats to minority indigenous groups which, they say, amounts to handing them to Morales, since he champions indigenous rights.

Morales' opponents also want a new electoral register ahead of the election, saying the current census is not reliable.

Bolivia's National Electoral Court has said it would take at least nine months to create a new register, which would imply the deferral of December's vote.

Across the landlocked country, hundreds of members of indigenous groups and trade unions joined the hunger strike in support of Morales, local media reported.

Bolivia, the poorest country in South America, has been racked by decades of political upheaval. The opposition is split ahead of December's vote, when Morales will stand for re-election and 166 lawmakers will be chosen.

Leading opposition party Podemos has yet to announce a presidential candidate. Several low-profile right-wing and center-left politicians plan to run against Morales.

According to a poll published in El Deber newspaper this week, some 54 percent of Bolivians think Morales will be re-elected, far ahead of his closest contender former President Carlos Mesa with 6 percent.

Morales, a critic of Washington and an ally of Venezuelan President Hugo Chavez, won sweeping victories in a recall vote in August and the constitutional referendum in January, showing strong backing for his leftist and pro-indigenous policies.