With returns in from almost 15% of polling stations in Sunday’s election, the 60-year-old Ortega had just above the 40% mark that would seal a first-round win and give him back the presidency 16 years after he was voted out at the end of a brutal civil war against US-backed Contra rebels.
If confirmed, it would be an embarrassing blow to the US, which fears the leftist would join an anti-US bloc in Latin America and warned of a cut in aid to Nicaragua under Ortega.
Conservative rival Eduardo Montealegre, Washington’s favoured candidate, trailed with 33.3%, although he insisted he won enough votes to force a run-off.
An independent quick count by a respected group said Ortega scored 38.5% and Montealegre trailed with 29.5%. Ortega needs either 40% of the vote, or 35% with a five-point lead, to take a first-round victory.
He would almost certainly lose if the race went to a second round, as Montealegre would pick up the votes of the third-place conservative candidate, Jose Rizo.
Montealegre refused to concede defeat and said the election was loaded with irregularities.
US officials in Nicaragua said they found irregularities in voting and refused to back the election until they were investigated.
It was Ortega’s third comeback attempt since 1990, when his Sandinista government was toppled by voters weary of a deep economic crisis and war against Contra rebels backed by the US.
“We have to leave behind the past, and move forward,” said Ortega’s vice presidential running mate Jaime Morales, a former Contra leader who joined his old enemy’s camp this year.
The US worries Ortega will team up with Venezuelan President Hugo Chavez and Cuban President Fidel Castro to challenge US interests in Latin America.