The Senate yesterday passed by unanimous voice vote a bill to name The Old Post Office complex the Nancy Hanks Center in memory of the former chairman of the National Endowment for the Arts who died Jan. 7.

The proposal, sent to the Senate by President Reagan, would retain the name "The Old Post Office" for the gray eminence of a building on Pennsylvania Avenue, in response to the concerns voiced by Washington history buffs. The Nancy Hanks Center includes the plaza on Pennsylvania Avenue and The Old Post Office, with its offices, clock/observation/bell tower, The Pavilion of shops and restaurants on the lower floors, and the performing arts stage in the lower atrium.

The House is expected to pass the bill and send it to Reagan to sign before the tribute to Hanks, which is scheduled for noon Wednesday at the Washington Cathedral. Musician Billy Taylor, a Martha Graham dancer, poet William Meredith and former Nixon arts czar Leonard Garment will participate in the memorial service, which will be open to the public.

Reagan sent a letter to Congress Wednesday urging the passage of the bill and saying:

"This designation is particularly apt since the renovation of the Old Post Office, its occupancy this year by federal cultural agencies and commercial enterprises and its exhibits are due in large measure to the foresightedness, persuasiveness, intellect and vigor of Nancy Hanks . . ."

Frank Hodsoll, current chairman of the NEA, supported the bill saying, "The center fulfills Nancy's dream of a multipurpose facility combining federal use with private enterprise while making cultural activities available to a wider public. It is a fine thing, indeed, for the president to recognize Nancy Hanks and the arts in this way on our nation's ceremonial avenue."

The bill, reported out of the Committee on the Environment and Public Works, includes a solution to a longstanding controversy. It provides that the National Park Service run the observation tower holding the Ditchley bells as well as the landmark clock. The observation tower, free to the public, is expected to be an attraction to compete with the Washington Monument.