President Evo Morales said Monday he was not abandoning hope despite indications Bolivians had rejected by a slim margin amending the constitution so he could run for a fourth straight term in 2019.
A member of the national electoral tribunal, Antonio Costas, said later that decisive official results from Sunday's referendum could not be promised for Monday.
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Morales said at a televised news conference he had faith in the slower-reporting countryside, where he has greater support.
"They don't like us much in the city," Morales said. In municipal elections last year, urban voters showed themselves more insistent on new blood in Bolivian politics and less tolerant of official corruption.
With 80 per cent of the ballots counted just after midnight Monday, the "no" vote stood at 55 per cent.
Unofficial quick counts by polling firms based on samplings of voting stations said 52 per cent voted "no." One pollster, Ipsos-Apoyo, counted ballots at one of every 15 polling stations.
The chief of the Organization of American States observer mission, former Dominican Republic President Leonel Fernandez, said there was no evidence of fraud but called the vote count slow.
Morales said he would respect voters' will, whatever the outcome. If the ballot question loses, life will go on, Bolivia's first indigenous president said. He blamed his disappointing showing on an opposition "smear campaign."
The vote's timing could not have been worse for Morales. He was stung this month by an influence-peddling scandal involving a former lover revealed by an opposition-aligned journalist and by a deadly incident of political violence.
In 10 years in office, Morales has presided over an unprecedented economic boom as prices for raw materials soared just as he took office.
He is credited with spreading Bolivia's natural resource wealth and empowering its indigenous majority. Gross domestic product per capita rose by nearly one-third, according to the International Monetary Fund, and a new indigenous middle class was born.
Export revenue down
But the boom is over.
Bolivia's revenues from natural gas and minerals, making up three-fourths of its exports, were down 32 per cent last year. Economists say Morales leaned heavily on extractive industries to pay for populist programs and failed to diversify the economy.
Analysts said the influence-peddling scandal clearly cost Morales.
It was revealed that a former Morales lover was named sales manager of a Chinese company in 2013 that has obtained nearly $500 million in mostly no-bid state contracts. The president denied any impropriety and claimed he last saw the woman in 2007. But a picture of the two together last year emerged, casting doubts.
The governing Movement Toward Socialism also has been wracked by scandal, including the skimming of millions from the government-managed Fondo Indigena, which runs agricultural and public works in the countryside.
Judicial corruption has been endemic and press freedom suffered as major news outlets were purchased by people friendly to the government. Critical media and environmentalists complained of harassment by the state.
Michael Shifter, president of the Inter-American Dialogue think-tank in Washington, called the tight vote a big blow to Morales, who tallied more than 60 per cent in his 2014 re-election.
"While few can deny that Bolivia has seen impressive economic growth and social progress under Morales' rule, many voters are sending a message that it is not enough," Shifter said. They want cleaner, more competitive politics, he said.
Morales, who entered politics as a coca growers union leader, could now be motivated to groom a successor, Shifter said.
At his news conference, the president said it was too early for that.