Mayoral front-runner Bill de Blasio Wednesday stood behind his work as a young activist in Sandinista-ruled Nicaragua as "the right thing to do."

His admiration of the Sandinista revolutionaries in the 1980s, when the Reagan administration denounced them as communists, was a viewpoint shared by many Americans at the time, he said at a Manhattan news conference where he received the endorsement of the Uniformed Firefighters Association.

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The Maryland-based social justice group de Blasio worked for was run by Jesuits and distributed medical supplies and clothing to "needy people who were obviously affected by the environment of war," the Democratic mayoral candidate said.

"The sense of injustice . . . was so obvious in terms of the United States' policies supporting regimes that were, in many cases, very unfair to their own people," de Blasio, 52, said. "That's why I got involved -- because I thought our policies were wrong."

De Blasio faced continued criticism Wednesday from Republican Joe Lhota, 58, who has likened his rival's activism and political philosophy to Marxism.

"Past is prologue; if you've done it before, you'll do it again," Lhota, a former MTA chairman, said Wednesday on WOR's John Gambling radio show. He added that a candidate's past matters "especially if we're gonna put children in their hands."

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With Matthew Chayes