Assistant Secretary of State Terence A. Todman, who has directed U.S. Latin American policy since the outset of the Carter administration, has asked to be replaced and is expected to be named ambassador to Spain, informed sources said yesterday.

If the appointment goes through, Todman, 52, a career foreign service officer, will be the first black to head what the State Department regards as a major, high-prestige embassy in Europe.

The sources said a final decision has not been made on Todman's successoras assistant secretary of inter-American affairs. But, they added, the frontrunning candidate is Viron P. Vaky, the U.S. ambassador in Venezuela.

During recent days, rumors about Todman's departure are known to have caused depending concern among many of his subordinates in the Latin American policy area. They fear that, even though he is leaving at his own request, the reasons for the change will be misunderstood in diplomatic circles.

In particular, many sources say, there is concern that Latin governments will conclude that Todman was oushed out because of allegations that he is out of step with the administration's human rights policy.

In the past few weeks, Todman increasingly has become the target of rights activists, who charged that he has little sympathy for the goals of the human rights policy and has acted instead as an apologist for such Latin American dictatorships as the regimes in Chile and Nicaragua.

Two Washington-based rights organizations supported by liberal church and political organizations, the Washington Office on Latin America and the Council on Hemispheric Affairs recently published lengthy criticisms of Todman for allegedly advocating on overly soft approach to Nicaragu's dictator, Anastsaio Somoza.

A central issue in these charges was a speech Todman made to a business group in New York on Feb. 14. In it, he listed 10 "don'ts" that he said should be avoided in implementing human rights policy.

As the rights activists were quick to point out, some of his points - not expecting Latin governments to change over night, believing that "only the opposition speaks the truth," condemning an entire government for acts of individual officials, withholding humanitarian aid from poor people - echoed the arguments frequently made by repressive regimes to justify themselves.

Despite the criticism, administration sources insists that Todman's approach - described by some sources as "moderate, pragmatic and designed to achieve practical results" - is fully in line with the president's goals. What's more, the sources said, the White House several times has made clear to Todman its approval of his implementation of human rights policy.

Instead, the sources said, Todman asked to be relieved, partly because of personal reasons. Some said that, as a career officer now in his 50s, he wants to hold a major ambassadorship and was concerned about being locked into the Latin American post for too long.

In addition, the sources added, Todman reportedly has become discouraged by the low priority the administration lately has given to Latin American affairs.

Although Carter talked a lot about upgrading Latin American relations during the early days of his presidency, the administration's attention increasingly has been focused on other parts of the world, with Latin America being pushed onto the back burner.

Latin American goverments have been especially concerned about various politically inspired protectionist moves by the administration - among them recent steep rises in sugar tariffs - that they regard as harming their goal of stable markets and prices for their raw materials exports.

That has caused many Latin governments, and their ambassadors here, to complain openly that Todman and the inter-American affairs bureau lack influence within the administration and are unable to impress on the president their needs and concerns.

Before being named assistant secretary early in 1977, Todman, a native of the Virgin Islands, had aerved as ambassador to Chad, Guinea and Costa Rica.