WE RETURN to a theme that has begun to haunt, even obsess us: the effect of the federal budget cuts on those called "the working poor." Let's start by trying to give some meaning to that mind-deadener of a designation, "working poor," which has its shorthand uses when you are talking about the economy, but which altogether omits the reality of the people it is meant to describe. That reality is poignant; and the way a government sees fit to approach it raises true moral issues, not just political or economic ones.

Here are people trying, by means of often hard and disagreeable jobs, to work their way out of dependency, which means off of welfare. They are doing this for a wage that is likely to represent only a very small amount more, if not in fact less, than they could get by merely staying home and taking a welfare check. There must be great motivation, determination, optimism and an instinct and appetite for dignity here. Why else would you go to the trouble, when you could be almost as well off by not working at all?

In recognition of this circumstance, government has contrived certain methods of encouraging and even partially subsidizing such work-effort. The point was to help people along with a gradually diminishing benefit, so that by taking a job they would not be 1)trading in a life of no work for one of work at no appreciable gain or 2)actually penalized for having taken work by the loss of health care and other assistance that goes automatically to welfare recipients. Avoiding the first of these conditions involves not deducting from a welfare payment one dollar in benefits for every dollar earned, but rather deducting somewhat less--say 67 cents--on a scale that comes up to one dollar very quickly, but which also makes taking a job pay. The second involves not automatically terminating health assistance, food stamps and the rest for those who do choose to work.

President Reagan, in his press conference yesterday, repeated his position that the thing should work pretty much the other way around--that welfare money should not go to those with other income. And the Congress has sustained him in this, making the legislative changes necessary to put the policy into effect. It is incredible to us that in the name of helping only the truly needy and getting the undeserving off the welfare rolls, the national government should have created rules that are bound to have precisely the opposite effect.

Did you read, in Thursday's paper, the accounts of the impact of all this on the working poor? We'll refresh your memory on one example. There is the predicament of the 36-year-old Sioux City woman earning $722 a month, whose son, in an after-school job, earns $120 a month. Until now, she and her five children got an additional $481 in welfare help and were eligible for Medicaid. Not exactly a Getty-sized monthly income, but here's what's happening to it now: under the new dispensation unless she and her son quit their jobs, they will no longer get the welfare aid or the Medicaid help. However, if they do quit their jobs, they will get on welfare $516 a month (about their take-home pay now) plus Medicaid benefits and $250 a month worth of food stamps. What would you do if you were the woman in that case?

Across the board it is the people in this niche of the society--those struggling to make it in a really cold and difficult world--who are most devastatingly hit by the new rules. In a day and age when everything often seems to be so murky and it is possible to see merit on so many sides of a dispute, we think this one is blessedly unambiguous. The administration's whole philosophy, the Congress' whole reputation are rendered suspect and scandalous by what they have done. Never mind that it now seems too late to undo the damage--until that damage has been undone, it will be impossible to take the rest of the enterprise seriously.