AMERICAN PERFORMANCE in exporting to foreign markets has weekened over the past decade. Confronted with fierce competition from other industrial nations, the United States is rightly concerned about raising exports. President Carter is contemplating a reorganization of the federal agencies that watch over foreign trade and encourage it. He has promised Congress a plan by July 10. Sens. William V. Roth (R-Del.) and Abraham A. Ribicoff (D-Conn.) propose a new Department of Trade to unify, coordinate, support, promote - and so forth.

Federal responsibilities for trade are about to be greatly expanded, and reorganization is a good and necessary idea. But to form still another new department is altogether unnecessary and therefore a bad idea.

The government's role in trade will grow because of the trade legislation Congress will take up later this summer. That legislation is the product of five years of negotiation in Geneva with nearly 100 other governments, and it attempts to establish a new and complex set of rules of international commerce.

The president's special trade representative, Robert S. Strauss, has said repeatedly that the value of these codes will depend entirely upon the skill and energy with which the U.S. government administers them. The Office of the Special Trade Representative has been a small staff loosely attached to the White House, and most of its people are there temporarily. In the future, it will need a permanent corps of highly trained and experienced specialists. But that would require only a couple of hundred people, not the thousands that staff even the newest of departments. More important, there is the danger that a new Department of Trade might rapidly turn into a Department of Protectionism.

The best answer is to strengthen the present Office of the Special Trade Representative, but to leave it attached to the White House where it is exposed to the broad sweep of national interests as the president, with his economic and diplomatic advisers, must deal with them. The research and promotional work are best left to the Commerce Department. American trade policy is much too important to be plunged now into the bureacratic warfare that attends the establishment of a new department.