Finding $2.6 billion in domestic trims. Be it legislative compromise or Gramm-Rudman, budget cuts are expected in social, mass-transit, and community programs. Feeding the hungry, meanwhile, is getting harder (P. 5).

Americans who are poor are suddenly confronted by the prospect of being in double jeopardy - of facing possible cuts in both private and governmental programs to aid people in need - in the aftermath of the ballyhooed ``summit'' in Washington that decreed cuts in this year's and next year's budgets. If Congress approves the summit proposals, an ``iffy'' prospect at present, the House and the Senate will have to find $2.6 billion in this year's budget for domestic social programs that it is willing to trim. Should Congress, however, turn down the compromise proposals, the poor would be no better off. Social programs would automatically be cut by that amount, as provisions of the Gramm-Rudman budget-slicing law take effect.

``I don't know where the cuts are going to be coming from'' to meet the $2.6 billion required by the budget summit compromise, says John Buttarazzi, a domestic policy analyst of the Heritage Foundation. When congressional committees ``actually sit down to write up the law to implement this, you're going to have every well-heeled lobbyist up there fighting for his or her program.''

Mr. Buttarazzi and others foresee the possibility that, in part because of high-pressure lobbying, Congress will be unable to agree on domestic trims. If that happens, the automatic budget-cutting provisions of Gramm-Rudman, now temporarily in effect, would become permanent.

It may be that the across-the-board trims that Gramm-Rudman would make in many social programs provide an outline of what Congress will do, if it is able to reach agreement. An analysis of Gramm-Rudman cuts prepared for the American Federation of State, County, and Municipal Employees finds that programs aiding poor children would be hit comparatively hard. It finds that nearly $350 million would be trimmed from programs set up to aid school-aged poor children, and an addition $100 million from the Head Start preschool program.

In addition, it concludes that some $250,000 would be cut from job training programs, $1 billion in federal highway aid, $266 million in mass transit assistance, $239 million in social-service block grants and $260 million in community-development block grants.

The programs of private agencies to assist the poorest of the poor are already in some danger, experts are beginning to warn. The ``single largest source'' of funds to private, nonprofit US organizations that feed and otherwise aid the poor is government, says Lester Salamon, director of the Johns Hopkins University Institute for Policy Studies. As federal funds have been cut back during the 1980s, these organizations have balanced their budgets by charging new fees for services, or by raising the level of fees already in place.

This development has ``profound implications ... for the future,'' Mr. Salamon says. ``This trend could suddenly shift the focus of the nonprofit sector away from those in greatest need to those able to pay, at least in part, for services.

Although gifts from private sources - foundations, individuals, and businesses - have risen substantially during this decade, Salamon says, they have made up only one-fourth of the deficit left by the government cutback; the higher fees produced the remainder.

Further, representatives of nonprofit agencies are deeply concerned that changes last year in the federal income tax law wil result in a significant decrease in private gifts to the nonprofit sector.