Ex-White House aide Oliver North - at the centre of a 1980s scheme to finance Contra rebels in Nicaragua - has warned the country not to return to the past.
Conservative Jose Rizo received Mr North's backing
Mr North said he hoped voters would not elect ex-Sandinista leader and former President Daniel Ortega - whom the rebels sought to topple.
Mr Ortega is the favourite to win the 5 November presidential elections.
His left-wing Sandinistas led Nicaragua in the 1980s and were voted out in 1990 after a decade of civil war.
Speaking in the capital, Managua, Mr North said Mr Ortega's election would be the worst thing that could happen to Nicaragua.
"I think that ought to have everyone concerned," he said.
"My hope is that the people of Nicaragua are not going to return to that. That's not good for your country. That's not good for my country."
Daniel Ortega is leading the polls
Mr North visited a memorial for fallen Contra rebels.
He also met conservative Jose Rizo, who according to polls is running third behind the conservative candidate favoured by Washington, Eduardo Montealegre.
The US ambassador in Managua, Paul Trivelli, said Mr North was not speaking for Washington.
Correspondents say that two decades after the Iran-Contra affair - through which secret arms sales to Tehran helped fund the Contras - Mr North remains a controversial figure in Nicaragua.
While Mr Rizo hailed him as "a person... who risked his political future for Nicaragua", others described his visit as "shameful".
The Iran-Contra scheme led to the prosecution, and conviction, of a number of Reagan administration officials.
Mr North - a lieutenant-colonel with the Marines - eventually saw his conviction overturned.