President Daniel Ortega is primed to win re-election on Sunday and become Nicaragua's first leader to serve consecutive terms since his Sandinista rebel army overthrew a right-wing dictatorship in 1979.

Ortega has won support over the last five years largely because he has overseen a period of economic progress, backed by financial aid from his socialist ally in Venezuela, President Hugo Chavez.

A former Marxist guerrilla commander and Cold War adversary of the United States, Ortega has solidified his hold on the Central American country with programs to improve health and education, microcredits and gifts of livestock.

He has a strong poll lead over a conservative opposition whose two main candidates failed to unite against him.

Ortega was allowed to run again thanks to a 2009 ruling by the Supreme Court, which is controlled by his Sandinista party. He has been credited with reducing inequality with social spending and allowing the private sector to operate freely, but also criticized for undermining democratic institutions.

Some critics fear Ortega aims to stay in power indefinitely like Chavez, whose petrodollars boost the Nicaraguan economy.

Ortega led Sandinista rebels in ousting the Somoza family dictatorship in the 1979 revolution and was the top figure in a government that withstood a U.S.-backed Contra rebellion throughout the 1980s.

He was elected president in 1984 but was voted out in 1990. He then spent 16 years as the main opposition leader before regaining power in a 2006 election.

Ortega has moderated his socialist rhetoric in recent years, although he is part of the Chavez-led bloc of left-wing governments in Latin America.

Backed by Venezuela, Ortega has cut poverty in this largely agrarian nation, and a recent CID-Gallup poll showed he was on course to win nearly half the votes in the election.

President Ortega has a big heart, said 46-year-old Migdalia Martinez, a mother of five selling sweets in Granada, an hour's drive from Managua.

The people are grateful. He has helped a lot of poor people who don't have roofs over their heads, said Martinez, who received a title to her home from Ortega two weeks ago.

Ortega's main challengers are radio personality Fabio Gadea and former president Arnoldo Aleman. Yet both refused to step aside to avoid splitting the conservative vote.

To avoid a second round run-off, Ortega needs at least 40 percent of the vote on Sunday, or 35 percent and a five percentage point lead over his closest rival.

DEVOUT CHRISTIAN

Ortega's relations with Washington remain tense and the U.S. government said it is concerned about irregularities ahead of the election, including difficulties some voters had to register and some domestic observers to get accreditation.

Ivan Lara, a 53-year-old business administrator planning to vote for Gadea, said Ortega could not be trusted.

Gadea is an honest man, he said. The other candidates are corrupt. For example, Daniel Ortega is a corrupt man because he is running as a candidate who does not respect the constitution that prohibits consecutive re-election.

Ortega has pledged to extend anti-poverty programs that include giving cows, pigs and hens to rural families, and analysts say he has no need to rig the vote.

Poverty has fallen to 57 percent of the population from 65.5 percent in 2005, according to government and World Bank statistics. Despite that, Nicaragua is still second only to Haiti as the poorest country in the Western Hemisphere.

Ortega's prospects might not look so bright if it were not for the support from Chavez, which analysts estimate brings in as much as $500 million a year, or 7 percent of gross domestic product.

The deal with Venezuela grants Nicaragua preferential access to oil, allowing it to weather price spikes that have hit poor neighbours like Honduras.

(Additional reporting by Alex Leff and Sean Mattson; Editing by Kieran Murray)