CAPITALISM in the United States no longer faces much of a challenge from international communism. So the hope of some reformers is that public attention will eventually shift to economic injustice within this country.Wall Street economist Sam Nakagama speaks of the "tremendous internal problems" that the nation has not dealt with adequately, partially because of the cold war. He hopes that the nation will now tackle more seriously such matters as inadequate education, the physical decay of cities, poor infrastructure, and many grim social problems. Citing one such social problem, Daniel Lichter, a sociologist at Pennsylvania State University, says it is "almost immoral" that in such an affluent society about 45 percent of black children come from families defined by government statisticians as poor - without sufficient income to provide an adequate level of food, clothing, shelter. Nearly 20 percent of all children are poor. Mary Jo Bane and David Ellwood, two Harvard professors, draw attention in the latest issue of the Harvard Business Review to the fact that "for 8 million American workers - roughly 7 percent of the entire US labor force - the experience of work isn't empowerment but quite literally impoverishment." They see poverty affecting the competitiveness of US business. Competitiveness isn't much related to the less than 10 percent of the poor living in ghettos, outside the economic mainstream and socially segregated, they note. "The existence of such an underclass may represent a long-term problem for society, but it's not easy to see why it jeopardizes the competitive performance of US companies." The issue is related to the half of the adult poor who work. "These people are poor not because they are unable or unwilling to work," the two write. "They are poor because they work at jobs that do not pay. Another large portion of the adult poor are single mothers caught in a trap between welfare and jobs that do not pay enough to make leaving welfare a rational alternative. And a full 40 percent of the poor are children - the work force of tomorrow." Dr. Lichter and a university colleague, David Eggebeen, have just written a paper that points to a particularly "troubling" trend in the 1980s - rich kids got richer and poor kids got poorer. 'Yuppie puppies," as Lichter calls them, are children of the richest 20 percent of families. "These kids are spoiled," he says. "They are cornucopia kids." After inflation, their family incomes rose from an average $70,900 in 1979 to $76,200 in 1989. As for children of the poorest 20 percent of families, their incomes sank on average from $10,900 to $9,000 in the same period. These poor children, when they go to school, go shopping, or watch television, are more likely to feel a sense of material deprivation than was the case 20 or 30 years ago, says Lichter. "It is survival of the fittest in this country," he says. "Those less fortunate are often left behind by current economic change." He wonders about the implications for the future from so many families and their children dropping back economically, socially, and culturally. What should be done? Professors Bane and Ellwood endorse some proposals of the 1987 National Commission on Children that would retreat from a welfare strategy and focus on reinforcing traditional values of work and family. One proposal aims at making work pay better for the working poor by enlarging the existing earned income tax credit. It is today equivalent to a 14 percent pay raise for the working poor. To further assist children, the Commission recommended a type of children's allowance, of $1,000 per child, be paid to the family of every child. These measures, reckon Bane and Ellwood, would get most two-parent, working-poor families out of poverty. For helping single-parent poor families, they suggest a comprehensive system of child-support enforcement and insurance. They figure a universal system could collect an additional $25 billion to $30 billion from absent fathers. Finally, they talk about guaranteeing health insurance coverage for all children. In other words, such critics are saying, the US needs more social measures socialism" in West European terminology.