President Reagan's proposed budget cuts would eliminate as separate entities some 40 federal social programs, many small but all aimed at areas of acknowledged national need -- and all with constituencies ready to spring to their defense.

The targets range from black lung clinics for Applachian miners to support for foster care, adoption of homeless children, fluoridation, migrant health, birth control, syphilis control, community health centers in ill-served rural and inner-city areas and alcohol and drug-treatment centers.

Rosalynn Carter's mental health program would be wiped out as a separate unit, but Nancy Reagan's foster grandparent program would not.

The 40 programs, all created with earmarked federal funds because the states were unwilling or unable to finance them, would be merged under Reagan proposals into four big "block grant" programs with an overall cut of about 25 percent from the $9.3 billion now being spent.

The states then could allocate the money any way they wish within the general purpose of the grant, ignoring some of the programs for which funds were previously earmarked and spending the entire amount on some other purpose.

Reagan says this flexibility to spend money more or less as they wish would allow the states to meet people's needs better without federal red tape. But critics say the plan would allow states to ignore many of the needs for which the federal funds were provided originally, particularly if the Reagan plans, when unveiled, let the states reduce their own level of financial input while the federal outlay is being cut 25 percent.

"Some elderly will go unfed. Some mothers receiving day-care services who are in jobs or training will slip back into welfare," said Jack Calhoun of the Child Welfare League office here.

The cuts in antidrug funding will "grind New York's fight against drugs to a near halt, and possibly force thousands of addicts and users into the streets," said Julio Martinez of the New York state division of substance-abuse services.

According to draft plans for the block grants obtained by The Washington Post, the 40 programs would be put into four separate block grants.

One would be a preventive health grant, totaling $266 million.

A second would consolididate general health services programs into a $1.154 billion grant.

A third, consisting of only two programs, low-income energy assistance and emergency welfare assistance, would total $1.4 billion a year and would be called emergency assistance.

The fourth, social services, would include as its two biggest merged units the present social services and handicap rehabilitation grant programs, and would total in the neighborhood of $4 billion.

The total of the four would be about $6.8 billion, or some $2.5 billion under the combined present outlay. Sources said these details may be changed before the final draft is approved.

The major programs slated to go into the block grants, with dollar amounts for fiscal 1982 if there were no 25 percent cut:

Social Services -- By far the biggest program scheduled to be put into a block, this is already a catchall outlay to the states, $3.1 billion distributed on a population basis. About 5 million people receive services of various types.

A fifth of the money is now used for day care for the children of working parents, ususally low-income, and almost a million children a year receive such day-care services. The program also funds senior centers, visiting nurses for 400,000 people each quarter year, mainly elderly and sick unable to leave home, education and job training for 800,000, meals on wheels for 324,000.

Child Welfare -- This program, at $525 million, enables states and local agencies to rescue children suffering physical abuse and neglect, place them in foster homes, group homes, shelters, aid in adoption and family counseling. A new law allows people who adopt handicapped and other hard-to-place children to receive welfare support for them.

Child-Abuse and Runaway Youth -- Provide $7 million and $10 million, respectively to the states for projects to prevent sexual and other abuse of children, keep about $43,000 runaway youths in homes or shelters and help reunite them with their families.

Developmental Disabilities -- Seed money ($51 million) for special projects on cerebral palsy, mental retardation and so forth.

Low-Income Energy Assistance -- $1.850 billion to help about $10 million low-income people with big bills caused by sharp rises in fuel costs. There is a hint in the draft paper that this program may be killed eventually, and a statement that distribution of the money next year should simply be based on how much each state gets now.

Rehabilitation Services -- Provides $1 billion a year for medical services and job training of 1.4 million mentally and physically handicapped people, three-quarters of them from families below the poverty line.

Community Health -- $335 million finances 573 rural and 299 inner-city community health centers in areas where health service of any type wouldn't be available because doctors don't like practice there. Five million people a year, mostly poor, use these centers, some paying for service, poorer ones getting it free or under Medicaid.

Black Lung -- $4 million funds 56 centers, treating 31,000 victims a year, mainly in Appalachia.

Migrants -- About 600,000 migrant farm workers get medical treatment, infant shots, at 122 migrant health centers financed by $44 million.

Maternal and Child Health -- About 2.5 million children, 68,000 infants and 363,000 mothers are treated annually in federally funded, locally run clinics for low-income people. This costs $355 million a year. For another $33 million, about 150,000 handicapped children, mostly preschool, receive special medical and related services.

Mental Health -- 2.5 million people, half with incomes under $5,000 a year, receive mental-health treatment, psychotherapy and the like at 750 local centers receiving $357 million in U.S. funding. This is the source of one-quarter of all the country's mental health care.

Alcohol, Drug Abuse -- $140 million a year helps fund local treatment centers aiding 351,000 alcoholics, and $230 million helps fund methadone maintenance and other drug treatment for 235,000 addicts at local health centers.

Family Planning -- The block grants also are expected to include the family-planning program, which helps fund contraceptive advice and devices for 4 million women (1 million of them teen-agers) at a cost of $165 million a year.

Others -- Also expected to be folded in are grants to combat leprosy in Hawaii, sudden infant death syndrome, genetic counseling for people carrying hereditary diseases, services for hemophiliacs, venereal disease screening and programs estimated to forestall 240,000 cases of syphilis and gonorrhea a year, aid for fluoridation, rat control, adolescent pregnancy services, immunization for millions of low-income children, high-blood pressure screening. Volunteers in Service to America (VISTA), community services and juvenile justice.