AMERICA is reneging on a consensus regarding welfare reached in the '80s. At that time policymakers at both the federal and state levels entered into a kind of compact on assistance to the poor.Under the agreement - reflected in state programs and the federal Family Support Act - government would maintain payment levels while providing more of the training and support services needed to promote self-sufficiency. In return, government could impose reasonable work and education obligations. That was before the recession. Today, faced with widening deficits, many states are freezing or slashing welfare payments. Some are attaching to the payments "behavior modification" conditions more demanding than work and school requirements. California wants to do both: cut payments and attach tougher conditions. Under a referendum proposal from Gov. Pete Wilson intended for next November's ballot, the state could lower welfare benefits by as much as 25 percent. Poor mothers who have more children while on welfare would be denied extra benefits. Teenage recipients of welfare payments would have to live with their parents. California's plan reflects a growing national mood. Nine states this year have reduced payments under Aid to Families with Dependent Children, the main federal-state assistance program for poor families, and 31 other states have frozen AFDC benefits. At the same time, states are trying to create incentives aimed at keeping young recipients in school, preventing further pregnancies, and promoting marriage, rent payment, and vaccination of poor children. Yet there is little evidence these kinds of behavior can be induced by manipulating subsistence benefits to already desperately poor people. Especially worrisome are payment strings that would penalize innocent children, such as refusing to increase benefits for additional children. Behavior-modification conditions in welfare programs could end up simply reducing benefit levels without making any significant inroads against the underlying causes of poverty. It can't be denied that states face severe budget shortfalls. Yet welfare comprises, on average, only about 5 percent of state budgets. From 1989 to 1991, AFDC payments declined as a percentage of spending in two-thirds of the states. Surely there are ways to bring state budgets into balance without taking a cruel toll on the neediest Americans. Cuts in welfare payments are doubly painful for recipients in the midst of a recession, when employment alternatives to state assistance are scarce.