Although President Ronald Reagan cut major domestic programs when he first came to office, since then congressional Democrats have been winning incremental, sometimes little-noticed, but significant expansions in social welfare programs for low-income parents and children.

Last year, for example, the budget package included an important expansion of Medicaid coverage for low-income children and a large boost in the income subsidy that the government pays to low-income working families with children.

As Congress opened this year, Democratic leaders like Senate Finance Committee Chairman Lloyd Bentsen (Tex.), Senate Majority Leader George J. Mitchell (Maine) and Rep. Henry A. Waxman (Calif.), who heads the House subcommittee on Medicaid, gave every indication that they will continue to press the same approach -- few sweeping overhauls of social programs, but targeted changes to fill in gaps and broaden coverage, aimed at helping the poor.

Already, Bentsen has introduced a bill designed to improve child-welfare programs, allow the states to further expand Medicaid for children and establish comprehensive new anti-drug programs for pregnant women and mothers with children. Preliminary cost estimates run in the range of $3.5 billion over five years.

The bill comes with the right endorsements: Mitchell and Finance Committee welfare expert Daniel Patrick Moynihan (D-N.Y.) are among the cosponsors. Mitchell called the bill a priority for this Congress.

The bill makes dozens of changes in state and local government programs to look after abused and orphaned children, try to protect them from drug abuse, avert family breakup and guarantee health care.

"In 1989 there were more than 2.4 million reports of child abuse and neglect," Bentsen said, "more than twice the number reported in 1980." He said the number of children who had to be placed in foster care, indicating serious dysfunction in their families, had ballooned to 360,000.

"The children entering the child-welfare system today are not only more numerous, they are more troubled and more seriously abused than a decade or even five years ago," he said. Drug and alcohol abuse among pregnant women and parents of young children are the major explanations for this trend, according to experts.

To address these problems, which also have been a major source of concern to Reps. Thomas J. Downey (D-N.Y.) and George Miller (D-Calif.), the Bentsen bill would provide more than $1.6 billion over the next five years in matching funds for "innovative preventive services."

The idea is to keep families together and prevent alcoholism, child abuse, drug abuse and neglect instead of waiting until these conditions have destroyed the family and done tremendous damage to the child, who then may be shuffled from one temporary caretaker to another.

As an example of the kind of programs to be funded, Bentsen said Maryland has set up Family Support Centers in low-income communities "where young men and women who want help in becoming better parents can come on a voluntary basis to get a variety of services -- health and nutrition counseling, encouragement and assistance in completing their education, training in job skills and, perhaps most important, guidance in child-rearing skills."

Another provision grants a tax deduction of up to $3,000 for the one-time costs of adopting a child with physical, mental or emotional disabilities.

A major provision of the bill would authorize $475 million over five years in matching funds to pay for comprehensive anti-drug and alcohol programs for low-income pregnant women and parents with young children, combining preventive services, treatment and follow-up services and including coordination with health care, to be paid for separately with general Medicaid funds.

A separate provision would permit states at their option to give Medicaid to children through age 18 in families whose income is below 185 percent of the poverty line -- a far higher income limit than currently allowed for most children.

In an interview last week, Waxman said that in the House he will press for a wide range of Medicaid improvements because "the Medicaid program is severely underfunded and does not cover most of the poor."

He said that in addition to the Bentsen proposal he would like to make early intervention drugs available under Medicaid to people who test positive for the HIV virus but who do not have AIDS and who would not otherwise qualify for Medicaid. If you wait until the disease breaks out and the drugs become available under normal Medicaid rules, it is too late to use an early intervention strategy, he said.

In addition, Waxman wants to mandate that the states provide Medicaid to pregnant women and infants with family incomes below 185 percent of the poverty line. He also favors providing care of the mentally retarded locally rather than in centrally located institutions.

Another proposal -- one that is not of an incremental nature, but much more sweeping -- is a new version of the parental leave measure sponsored by Sen. Christopher J. Dodd (D-Conn.) and vetoed by President Bush last year. It would force employers with 50 or more workers to grant 12 weeks a year of unpaid leave, with a job guaranteed on return, to workers employed at least one year who need time off to have children or care for an ill member of the family. Like the Bentsen bill, this measure is cosponsored by Mitchell, who called it "a high priority for this Congress."