A congressional effort is under way to investigate the lobbying by the Pentagon and Lockheed on behalf of the C5 transport plane.

Sen. William Proxmire (D-Wis.) said yesterday that he has asked the General Accounting Office to look into the matter, while Rep. Norman D. Dicks (D-Wash.) is sponsoring a House resolution demanding that the Defense Department supply Congress with details about its involvement in the C5 campaign.

Proxmire, ranking Democrat on the Senate Appropriations Committee, said "the existence of this lobbying team not only runs counter to the clearly stated vote in the Senate but also may violate U.S. law" forbidding the use of government money to influence members of Congress.

Proxmire noted that several military officers were on the team trying to reverse the Senate vote against buying the Lockheed C5 as the military's new long-distance transport. The Senate said the Boeing 747 should be purchased for this role.

Dicks, who represents Boeing's home area around Seattle, wrote Defense Secretary Caspar W. Weinberger on June 25 that the printout of the Lockheed lobbying plan for the C5 "clearly indicates that Department of Defense officials have exceeded their legitimate role . . . . " Dicks' resolution, pending in the House Armed Services Committee, calls for the Pentagon to disclose to Congress the full details on the lobbying effort, including who participated in it.