Word at the State Department is that counselor Robert B. Zoellick, a key member of James A. Baker III's closely knit brain trust, is going to be wearing a second hat soon, taking on the job of undersecretary for economic affairs.

Zoellick, 37, won't be giving up his current job as counselor, sources said, so there will be no scramble for a replacement. It may be, however, that he will be spending less time on Baker's plane, where he, policy planning director Dennis B. Ross and spokesman Margaret Tutwiler are the key troika of advisers.

As counselor, a loosely defined job, Zoellick's role has been free-floating, doing whatever Baker wants. As a result, he has been perhaps the widest ranging policy adviser, involved in just about every area of foreign policy -- from Nicaragua to Cambodia, from the Mideast to German unification to the Soviet Union.

That range may be somewhat more limited if Baker gives his final approval to the move. But the feeling among a number of department watchers and insiders is that State, traditionally eclipsed by Treasury on economic matters, has been on the decline of late, especially in Latin America.

Richard T. McCormack, who now holds the job, has never enjoyed similar access to Baker. Department sources said McCormack, a former aide to Sen. Jesse Helms (R-N.C.), got the job last year as a gesture toward Republican conservatives. McCormack announced last month that he was leaving in the next few months.

Zoellick, a former assistant to Budget Director Richard G. Darman when Darman was at Treasury, is seen as someone who, at interagency meetings on economic matters, can speak with the clout that is needed to make sure State's viewpoint is heard. Zoellick also worked for Baker at Treasury in 1988.

If confirmed by the Senate, Zoellick would take over a post that since its creation in 1958 has been held by such notable figures in the business and economic world as Wall Street tycoon C. Douglas Dillon, Washington lawyer William D. Rogers and Harvard economist Richard N. Cooper. Former CIA director William J. Casey had the job for a time during the Nixon administration.

Recently, Zoellick has spent the greater portion of his time on economic issues, handling among other things, relations with the European Community, the Mexico free-trade arrangement, the Asia-Pacific Economic Cooperation group, and questions involving the economic situation in the Soviet Union.

Sources at the department say Zoellick, in keeping the counselor job, will continue to oversee the paper flow to Baker, therefore keeping an eye, and, if he wants, a hand, on everything going on.