President Bush said yesterday he would nominate Rep. Lynn Martin (R-Ill.), a longtime political supporter who occasionally opposed him on key social issues, to be secretary of labor.

In a brief announcement as he headed for Camp David, Bush called Martin a "cherished friend" and "unofficial adviser."

As labor secretary, Martin would be charged with enforcing for more than 100 million Americans workplace rules ranging from the security of pension plans and payment of minimum wages to job site safety and equal employment opportunity for federal contractors.

Over the last 20 years, the Labor Department has evolved into one of the largest regulatory agencies in the federal government. Labor-management relations, once the department's focus, take little of its time today.

Perhaps the most immediate issue facing the new secretary is the "glass-ceiling initiative" to end discrimination in management ranks of major corporations. The initiative was put on hold when Secretary Elizabeth Hanford Dole resigned last month to become president of the American Red Cross.

Bush said he and Martin will meet at the White House Monday to discuss the "direction of the Department of Labor."

News of her appointment was greeted coolly by organized labor. In a statement, AFL-CIO President Lane Kirkland questioned her commitment to workers and their families, "particularly when the economy is in recession and workers bear the brunt of hard times. . . . Her voting record has not reflected a sensitivity to the needs of workers."

Sen. Edward M. Kennedy (D-Mass.), chairman of the Labor and Human Resources Committee, which will consider Martin's nomination before it goes to the floor, said, "I commend Lynn Martin on her nomination and I look forward to her confirmation hearing." He added that the Labor Department needs a strong secretary to "stand up for hard-pressed families against the pro-business tilt of the White House."

Bush telephoned Martin, 50, at her Illinois house to offer the Cabinet post just moments before announcing her appointment. White House Chief of Staff John H. Sununu had talked with her earlier in the day to tell her she had the job, a source close to Martin said.

Martin issued a statement from her congressional office saying she was honored to be nominated and thanking the president for his support.

A week after Martin lost her bid to unseat Sen. Paul Simon (D-Ill.) last month by nearly a million votes, she telephoned Bush to seek the labor job. That was her last conversation with the president or anyone on his staff until yesterday, according to administration sources.

"It's a political payoff" for Martin giving up a safe House seat to take on Simon, a source familiar with the selection process at the White House said.

After 10 years in the House seat once held by Rep. John B. Anderson (R-Ill.), Martin has a reputation for being very conservative on fiscal matters and more moderate on social issues. She supports abortion rights and has voted for the Equal Rights Amendment. In the last Congress she voted for the civil rights and parental leave bills, both vetoed by Bush.

Martin has described herself to staff members of the House Budget Committee as conservative on economic issues, and she is an ardent supporter of the White House on key budget matters.

One source said yesterday that Sununu told Martin she would not be allowed to pick her own deputy, who essentially runs the department day to day. "He told her she was going to have a deputy that was going to know the department," the source said.

Although she has a reputation as an aggressive partisan in Congress, Martin also is reported to have good personal relationships with Hill Democrats, a trait that sometimes rankles her Republican colleagues.

Whatever her standing with fellow Republicans in the House, however, Martin is reported to have a good relationship with Bush, particularly after the 1984 presidential election campaign, when she served as a stand-in for Geraldine A. Ferraro during rehearsal for his debate with the Democratic vice presidential nominee.

The mother of two and a Roman Catholic, Martin has been involved in a commuter marriage since 1985 when she and U.S. District Judge Harry D. Leinenweber of Chicago were wed.