Ortega is seeking a second consecutive term, having taken office in January 2007, The New York Times reported. He previously served as president from 1985 to 1990.
Ortega was supported by 48 percent of those surveyed in a recent CID Gallup poll, 18 points ahead of his closest rival, Fabio Gadea, a 79-year-old radio personality.
"This democratic process is going in a good direction," Ortega's wife and spokeswoman, Rosario Murillo. "That is a reality you feel everywhere. There is a mood of celebration."
Although the president has boosted the country's economy and has policies aimed at the working class, foreign observers are worried about legitimacy, should Ortega win a second term, the newspaper reported.
Ortega changed the Nicaraguan Constitution in 2009 to permit his re-election. Previously, leaders could not serve consecutive terms and were limited to two terms in all. Nicaragua's Supreme Court, which Ortega controls, ruled that the limits on presidential terms violated human rights.
"If the candidate is in the people's interest, they will elect him, and if the candidate does not represent their wishes, they will not elect him," Ortega said in 2009. "Obstacles should not be set up to deny them that right."
Former American Ambassador to Nicaragua Robert J. Callahan said if there is evidence of widespread fraud in Sunday's election, the United States may cut off economic aid to the country.