There was little fanfare when the White House recently sent the name of Susan E. Phillips, an associate director of the Office of Presidential Personnel, to the Hill as its nominee to fill a seat on the Federal Trade Commission.

But on Dec. 4, the day the consumer subcommittee of the Senate Commerce Committee began to check into Phillips' background, Howard Phillips, president of the Conservative Caucus and the nominee's brother, called Reagan a "useful idiot for Soviet propaganda."

That remark apparently stirred some congressional interest in the nominee, while at the FTC there has been talk about whether she is qualified. Phillips, nominated to fill the seat of Patricia Price Bailey, would be the first who is neither a lawyer nor an economist, according to commission sources. The four present commissioners and chairman, Daniel Oliver, are attorneys.

Before joining the White House staff in 1985, Phillips directed the Institute of Museum Services in 1983-85. She also served as director of intergovernmental and interagency affairs at the Education Department in 1982-83. She worked for the Conservative Caucus in 1976-82. New Faces . . .

President Reagan has appointed Charles N. Rostow, deputy legal adviser to the National Security Council, as his special assistant for national security affairs. Rostow, who will succeed Paul Stevens, will also serve as the NSC's legal adviser. Rostow's father is Eugene Rostow, who served in the Johnson administration and as Reagan's first director of the Arms Control and Disarmament Agency.

Mark Bisnow has joined the Agency for International Development as special counsel, where he will lead a task force on financial management practices.