Democrat Jesse L. Jackson said yesterday that Nicaragua may be justified in planning a major military buildup, even as Central American leaders are working toward a peace plan, because of continued U.S. support of the contras and a threat of invasion.

"That {buildup} plan could be in anticipation of an attack by the U.S. and more support to the contras," Jackson, a presidential candidate, said on CBS News' "Face the Nation."

"Those people are living under fear every day, fear of invasion from neighboring countries, financed by our governments, with reports that our government has financed ways to overthrow their government," Jackson said.

Nicaraguan Defense Minister Humberto Ortega's disclosure of a 15-year plan to build up the Sandinista regular armed forces, reserves and militias to a total strength of 600,000 by 1995 shocked many in Congress and strengthened President Reagan's hand in seeking additional aid for the contras.

Asked during yesterday's broadcast whether he was justifying a military buildup, Jackson replied: "Since I've been there, I understand really what the options are. If you have military bases built up on your borders, if you have your harbors mined, if you have your crops destroyed, that creates a tremendous psychological crisis."

On Gary Hart's surprise reentry to the Democratic presidential campaign last week, Jackson asserted that Hart is a "disruptive force" within the Democratic party.

"There is a fallacy for Gary Hart to project a superiority complex without superiority." Jackson said. ". . . I just hope we will survive this trouble and keep our eye on the prize."