Bolivia's ruling socialist party has defied the results of a February referendum and backed President Evo Morales for a fourth term in 2019.

The Movement for Socialism (MAS) party, holding a congress in the eastern city of Montero on Saturday, approved the candidacy of Morales, Bolivia's first president with an indigenous background, in a unanimous vote.

Morales welcomed the party's decision, saying, "If the people decide it, Evo will continue."

He added: "So many times, we have defeated the right."

Morales was first elected president in 2005, and re-elected in 2009 and 2014.

But he narrowly lost the referendum in February on the question of whether the constitution should be revised to permit him to run again in 2019. His current term expires on January 22, 2020.

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The party congress recommended "four legal alternatives" to allow his candidacy within the constitutional framework, according to a union leader who read the conclusions.

The first was a partial constitutional reform through an initiative requiring the signatures of some 20 percent of the electorate. Another also involves a constitutional reform to allow an extended presidential mandate.

The third recommends that the president renounce his office before the 2019 elections, so that he would not have served three full terms, while the fourth involves a reinterpretation of the constitution.

Morales, who has cultivated an "everyman" image, has been highly popular throughout most of his presidency.

He won his first election with 54 percent of the vote, his second with 64 percent and his third with 61 percent. 

Morales has generally benefited from a fragmented opposition.

At the time of the February referendum, his popularity had been damaged by allegations that he had fathered a child with a young woman, Gabriela Zapata, and done favours for the Chinese company employing her.

He admitted the two had had a son, who died in infancy, but emphatically denied the other allegations.

Still, he lost the referendum by a narrow margin, 51 percent to 49 percent, and vowed to continue pressing the leftist platform that underlies his popularity.

As president, he has worked to redistribute the nation's natural gas wealth and provide a more inclusive environment for the indigenous majority.

To burnish his "common man" credentials, Morales has often travelled on foot or stopped to play football with locals.

But some Bolivians said they felt he had amassed too much power in his years in power, and that the one-time outsider had himself joined the elite establishment.

In a survey early this month, the Ipsos polling firm found Morales's popularity had slipped a bit, but was still relatively solid, at 49 percent.

Source: News agencies