The District of Columbia, widely regarded as an affluent exception to the economic downturn felt by urban America, actually faces some problems sooner and worse than other cities, a national expert in government trends told a congressional hearing yesterday.

"The District, while unique because it's the nation's capital . . . is on the cutting edge of what's happening to other major cities," Ralph R. Widner, president of the Academy for Contemporary Problems, testified before the House District Committee.

Well-intentioned government programs have failed to provide jobs and a good life for many inner-city black residents, Widner said, leaving them isolated and alienated -- "outside the society as a whole . . . a powder keg, ignored at great risk to the society."

Widner's assertion drew agreement from the executive director of Washington's Southeast Neighborhood House, Laplois Ashford, who said the "utter hostility" and despair exhibited by jobless and financially squeezed Anacostia residents "is a potential for an urban Mount St. Helens."

"I am fearful. I saw the riots of the '60s," Ashford told Rep. Ronald V. Dellums (D-Calif.), who conducted the hearing. "I would not be shocked if something happened now."

Dellums, who represents a heavily black constituency in Oakland, agreed. "All hell is about to break loose in this country . . . if we don't deal with the problems of Washington, D.C., and the other cities," he declared.

Dellums said he was using his District Committee chairmanship in an attempt to focus national attention on urban problems at a time when the political tide is pulling the nation in a conservative, budget-balancing direction.

He told Moon Landrieu, secretary of the U.S. Department of Housing and Urban Development, who also testified, that the Carter administration's inflation-fighting policies are dangerously worsening joblessness among young blacks. Landrieu disagreed, saying such unemployment "is almost a constant in our society (that) grows worse in times of recession," and that inflation threatens the nation's existence.

Widner, the day's lead-off witness, heads a foundation sponsored by seven national organizations of state, county and city officials.

The District's chief problem, he said, is an inability to match its undereducated work force with white collar and service jobs that are available here -- a problem increasingly faced by other bit cities as manufacturing jobs are moved to suburban and rural locations.

Statistics show that 73 percent of the white men who work in the District have professional, technical or managerial jobs, compared with 17 percent of black men, he said. By contrast, 81 percent of all craftsmen are black, Widner reported.

Even though the District ranks second in the nation in per capita income when compared to the states, Widner said its poverty rate of 12.5 percent of the population exceeds the national average by 1.1 percent.

"While it may be hackneyed to say it," Widner told the committee, "the main solutions to this problem lie with our education and training programs and their relationship to the realities of the job market."