In his early years in the Senate, Richard S. Schweiker, the man Ronald Reagan has chosen to head the giant Department of Health and Human Services, used to score 100 percent in the vote ratings of the AFL CIO.

Even in 1976, when the conservative Reagan shocked the country by saying he would select Schweiker as his vice presidential running mate if nominated, Schweiker was rated the most liberal Republican senator by the AFL-CIO, for having sided with the unions on 88 of 96 key votes in his Senate career.

Since then, Schweiker has moved steadily toward more conservative positions. While he doesn't qualify as an arch-conservative, his labor ratings have plummeted and so has his standing with liberal organizations. In 1979, for example, he voted with the AFL-CIO on only nine of 18 key votes, and this year, on only seven of 19. The American Federation of Teachers rated him correct in only five of 11 key votes.

In the past three years he has voted in favor of school prayer, against liberal amendments to shift money from defense to domestic programs, in favor of relaxing some occupational safety requirements and capping Social Security disability benefits, against a key amendment expanding the food stamp program, in favor of tuition tax credits, against President Carter's hospital cost containment proposal.

Actually, even before the 1976 offer from Reagan, Schweiker always had a conservative underside beneath his liberal outer vestments.

He is a member of the Central Schwenkfelder Church, a tiny religious group that includes opposition to abortion among its tenets, and as a result, Schweiker has opposed abortion, except to save the life of the woman. In recent years, as senior Republican on the Senate Appropriations subcommittee on labor and health, education and welfare, he has been a leader in imposing restrictions on federally financed abortions for low-income women.

Similarly, though he has generally had a pro-civil rights record, he has had qualms about school busing and even before 1976 had sometimes voted against it. Moreover, he has always been opposed to gun control laws, and in fact used that issue to unseat then-Sen. Joseph Clark (D-Pa.) in 1968.

Although Schweiker has turned more conservative in recent years, he is no super right-winger. While he has often emphasized cutting fraud out of Medicare, Medicaid and Welfare -- three programs that will be under his jurisdiction at HHS -- he has sponsored, with Sen. Edward M. Kennedy (D-Mass.), a bill to foster health maintenance oganizations, and has frequently supported funds for health and education in the Appropriations Committee.

As secretary of health and human services, a post to which he brings enormous program knowledge because of his experience on the Hill, Schweiker will have jurisdiction over some of the biggest programs in the government. The department has 160,000 employes and a budget exceeding $200 billion a year.

The biggest programs are Social Security -- with 35 million beneficiaries getting nearly $100 billion a year in monthly old-age and disability checks -- Medicare, Medicaid, Aid to Families with Dependent Children and Supplemental Security Income (SSI). Schweiker aides said yesterday that he supports these basic programs and generally believes in their principles.

The department also is the major medical research and regulatory agency of the government.

Here is how Schweiker stands or has voted on some major issues involving these programs in recent years:

Social Security: In 1972, Schweiker voted for the 20 percent increase in benefits opposed by President Nixon, and for automatic cost-of-living increases each year in the future. In 1977 he initially voted against the tax bill restoring fiscal health to the system, but only, aides said, because the Senate version breached the traditional 50-50 tax split between employers and employes. When the bill came back from the House with the tax restored to the long-standing 50-50 form, Schweiker backed it.

National health insurance: He opposes comprehensive, government-financed national health insurance, but has introduced legislation to foster catastrophic coverage (protection against extra-high costs due to serious long-term illness) and to force employers to give workers a choice of plans with different coverage and costs. He didn't support the child health assurance bill to provide special services to low-income children.

Schweiker has opposed Medicaid antifraud legislation, and has said food safety laws need to be revised to eliminate absolute bans on some dangerous substances if benefits can be shown to outweigh risks.

Schweiker, born in Norristown, Pa., June 1, 1926, graduated Phi Beta Kappa from Penn State, was a businessman for 10 years before winning House election in 1960. He was elected to the Senate in 1968 and reelected with AFL-CIO support in 1974. His wife, the former Claire Coleman, with whom he has five children now ages 11 to 23, was the original "Miss Claire" of TV's Romper Room.

As a senator, Schweiker has been a fast talker, a good politican, a "quick study" and a man known for having a very good staff. At least two aides, his chief of staff Dave Newhall and health man Dave Winston, are expected to go with him to HHS.