James C. Miller III, the Office of Management and Budget official charged with overseeing the Reagan administration's regulatory program, has emerged as the leading contender to become chairman of the Federal Trade Commission, informed sources said yesterday.

Ironically, OMB has advocated stripping the FTC of a major piece of its authority by eliminating funding for the agency's antitrust activities and closing all 13 of the FTC's regional offices.

Although the administration later backed down from a proposal to close the FTC's Bureau of Competition, OMB officials have continued to argue for sharp cuts in the agency's budget.

As head of the administration's FTC transition team, Miller's report took a softer position on the agency, emphasizing that the agency should concentrate on the economic consequences of horizontal mergers, terminate cases based on "social theories" and set up tighter internal mechanisms for reviewing ongoing cases.

The group also recommended a 25 percent cut in the agency's budget for fiscal 1982 and closing of the regional offices. The Miller team also suggested that Reagan appoint a new chairman from outside the agency.

Miller said yesterday he would be interested in the job and "flattered" if President Reagan ultimately offered him the position. Sources said Miller is being considered for a term on the FTC opening up in the fall. There is also currently one other vacancy on the FTC.

Administration sources said the Miller appointment would be logical because most of the key facets of the Reagan regulatory program would be in place by September. "It's his if he wants it," said one well-placed official.

Under Miller, who is OMB administrator of information and regulatory affairs and executive director of Reagan's task force on regulatory relief, the OMB has moved aggressively to become the centerpiece of efforts to curtail and revamp a wide range of government regulatory activity.

The candidacy of Colorado Secretary of State Mary Estill Buchanan is no longer an option, as some congressional opposition to her nomination ended that bid. Others reportedly under consideration are Washington attorneys Edward Weidenfeld and Wayne Bishop.

Acting FTC Chairman David Clanton, a Republican nominated by Reagan to the interim position, also is interested in getting the permanent chairmanship. Sources say he has little support within the administration. The other Republican currently on the five-member panel is Patricia Bailey, who was on the verge of getting the acting chairmanship until a speech by her, critical of OMB cuts in the FTC's budget, came to the attention of administration officials.