In a move that some State Department sources call a bow to congressional supporters of Nicaraguan President Anastasio Somoza, the Carter administration is releasing up to $12 million in aid that had been held up for human rights reasons.

According to the sources, the policy reversal resulted, at least in part, from pressure by Rep. Charles Wilson (D-Tex). He is the most active and vocal congressional member of what anti-Somoza forces refer to as "the Nicaragua lobby."

Wilson, the sources said, had threatened an all-out effort to hold the administration's fiscal 1979 foreign aid request hostage to his demands.

Specifically, the sources added, Wilson had voted to use his influence in the House Appropriations Committee to block a requested increase in the U.S. contribution to international lending institutions.

He also threatened to sponsor legislation cutting off aid to at least six other countries that he said have records of human rights violations worse than that of Nocaragua.

Administration officials involved in the decision to free the aid concede privately that Wilson's pressures were "an influencing factor." However, they also insisted that several other considerations played a part.

All of the aid in question, these officials noted, is of a non-military nature and meets congressional and Carter administration guidelines about providing direct benefits for Nicaragua's poor and needy.

In addition, the officials stressed, the ban imposed by the administraion last year on transfers of military equipment to Nicaragua remains in force. They also said the administration had not agreed to a request by Wilson to provide Somoza such "gray area" equipment as helipcopters.

However, even clearly humanitarian aid is certain to generate controversy because Nicaragua, a tiny Central American country of 2.2 million, has become a symbol.

Nicaragua is where the Carter administration has used foreign aid most directly and overtly to influence the human rights situation. The aim has been to induce Somoza, whose family has controlled the country for 42 years, to loosen the dictatorial hold exercised by his military force, the national guard.

Last September, Deputy Secretary of State Warren M. Christopher, who oversees implementation of human rights policy, made two major policy decisions that department sources said were intended as a "signal" to Somoza.

Christopher allowed the signing of a $2.5 million arms credits authorization for Nicaragua, but stipulated that implementation of the agreement should be held in abeyance. Also, he ordered that $12 million in non-military loans and grants he held up.

In ensuing months, the administration continued to take a relatively tough line on aid to Nicaragua. In its fiscal '79 aid requests, it trimmed military assistance for Somoza's forces to only $150,000 in training grants.

Wilson has long been in the fore-front of those protesting the crackdown on aid to Nicaragua. He argues that Somoza is an old and proven ally who has beem made a whipping boy while Washington ignores rights violations by countries with greater strategic or economic importance.

The first signs that his lobbying was having an effect became evident two weeks ago when the State Department quietly released $160,000 in unexpended military credits to cover purchase of equipment for a Nicaraguan military hospital.

According to department sources, the request for the hospital equipment had been blocked for months by the Bureau of Inter-American Affairs. However, they said, that changed dramatically after Wilson called Christopher and several other top-ranking department officials to threaten action against other parts of the aid bill.

About the same time, they asserted, several actions were taken to start freeing the $12 million held up since last September.

A $3.5 million loan for rural nutrition activities already has been released. And, the sources said, if preliminary feasibility studies prove favorable, it is virtually certain that between $7.5 million and $8 million - most of the balance of the $12 million - also will be released in the form of an educational development loan.