By continuing to press for additional reductions in welfare and other forms of public assistance, the Reagan administration is "failing the moral test of government," D.C. Del. Walter Fauntroy said yesterday.

"Those at the command posts of power," Fauntroy told a public hearing organized by the Washington Urban League, "have concluded, 'who needs black people? who needs the poor, the young, the elderly, the sick?' "

But, Fauntroy warned, should this attitude continue to build, with its explicit bias toward the rich and the military establishment, "ten years from now, the world will be saying, 'Who needs the Americans?' "

The hearing, one of 15 held around the country, was a daylong affair intended to raise attention in Congress and among the public about the dangers the League believes are inherent in the Reagan administration's efforts to cut deeper into Aid to Families with Dependent Children, food stamps and other welfare programs.

Most of those who spoke said that any further reductions in aid to the disadvantaged would have the effect of further alienating them from the rest of society and making them more dependent than ever on welfare.

The administration's latest proposals would "push low-income families deeper into poverty and virtually eliminate any incentives for welfare mothers to work," said Cheryl Rogers, coauthor of a new study by the Center for the Study of Social Policy.

Disclaiming the president's promise of a "safety net" for the "truly needy," Rogers said that cuts the administration is now seeking would, for the first time, affect "families who are totally dependent on government assistance--those mothers with very young children who are unable to work, elderly and disabled persons, and adults who must care for an incapacitated spouse."

The working poor, those who hold low-paying jobs and manage to support themselves with minimal government assistance, will find their incomes reduced even more than those who don't work, according to Rogers.

"Nationwide," she said, "the total disposable income of the average working AFDC mother and two children would fall from $595 a month (101 percent of the poverty line) to $432 a month (or 73 percent of the poverty line) from cuts in AFDC and food stamps alone."

In 24 states, working mothers who receive welfare support would end up with less income than mothers who don't work and in 11 other states the working welfare mother would end up with just $1 a day more than the nonworking mother, Rogers said.

The net result of cuts in aid to working mothers would be to force many of them to give up their jobs, producing "a substantial increase in the federal government's costs," she said.

Lorthea Gray, a mother of three who lives at 3654 New Hampshire Ave. NW, said she holds two part-time jobs that provide her with a net income of $634 a month. Until last month, Gray said, she also received $130 a month in AFDC payments and $48 in food stamps. "Certainly, I was not getting rich off welfare," she said.

Now that her welfare payment has been eliminated, and with it Medicaid payments for treatment she requires for a kidney ailment and psychiatric care for one of her children, "I must give up one of my part-time jobs so that I will again qualify for AFDC. This is the option that has been given me by the same administration which said it wanted to help the working poor."