In 1973 the maximum cash payment for a welfare family of four in Texas with no other income was $140 a month.

The maximum payment today is a dollar higher, $141, even though consumer prices have doubled since then.

Texas is the most extreme example of a common phenomenon.

Many states let their welfare benefits lag behind inflation; in many states benefits are quite low. A government study says that nationally the purchasing power of the average maximum Aid to Families with Dependent Children (AFDC) benefit fell 18 percent from 1973 to 1979, from $424 a month to $349, measured in 1979 dollars.

It was partly to lift these benefits and help even out welfare levels nationally that Congress greatly expanded the all-federal food-stamp program in recent years. The less a household's cash income, the larger its food-stamp entitlement. Stamps have thus served as a welfare supplement, bringing recipients in low welfare states up toward the levels in states that are more generous.

And that is why many welfare experts are criticizing President Reagan's proposal to turn food stamps as well as the AFDC program over to the states.

The president would do this as part of his "New Federalism" plan, in return for federal assumption of the state share of the Medicaid program.

Some experts say they fear that, on their track record, states would let food-stamp payments fall off in the future, just as they have AFDC payments in the past. Benefits generally would sink, and the national benefit pattern become more uneven.

"No one should be under any illusions that benefit levels in these programs will be maintained by the states for any length of time," said Robert Greenstein, former administrator of the Agriculture Department's Food and Nutrition Service and now director of the privately funded Center on Budget and Policy Priorities. "It is likely that most states will cut back, and some are likely to cut back drastically."

Greenstein is a food-stamp proponent who might be expected to make such remarks, but conservatives also have been critical. Richard P. Nathan, assistant director of the Office of Management and Budget and undersecretary of the Department of Health, Education and Welfare in the Nixon administration and now at Princeton, has written of the Reagan plan:

"Turning welfare and food stamps back to the states goes against the grain of history. It could create a vicious competition among the states to push out and keep out the most vulnerable groups: working-age poor people and their children" by failing to provide adequate benefits.

The crazy-quilt character of welfare benefits across the country has been a problem in every effort to reform the program, including the one in which Nathan was involved centrally in the Nixon years.

The AFDC program, in which the federal government reimburses the states for anywhere from 50 percent to 77.5 percent of the cost of supporting families with minor dependent children, was created by the Social Security Act of 1935.

Each state can set its income cutoffs for eligibility and its benefit levels, and these vary greatly.

These variations far exceed regional differences in the cost of living. In Alabama, according to the Congressional Research Service, the maximum benefit for a family of four in 1981 was $148, although the state officially calculated that such a family needed $480 a month to live.

In California, by contrast, the maximum was $601 a month, the same amount as the state estimated was needed to live on. Although Texas and Louisiana border each other, Texas paid $141 and Louisiana $213. And a family living in New Hampshire could get $372, but would be better off in neighboring Vermont at $566.

"The inadequacy of state AFDC laws, especially in the South, was one of the chief reasons for enactment of food stamps," said John Kramer, associate dean of the Georgetown University Law School and an adviser to Rep. Frederick W. Richmond (D-N.Y.), who heads the House subcommittee with program jurisdiction. Kramer, as director of the National Council on Hunger, was one of the key figures in expansion of the food-stamp program in the late 1960s and 1970s.

Unlike AFDC, the food-stamp benefit is not set by the states but by the federal government, which also pays the entire bill, with uniform nationwide rules.

The amount of stamps received automatically goes up periodically to keep pace with increased food prices, so stamps are in effect "indexed" for inflation while AFDC is not. There is a uniform national income level for eligibility, most recently set at 130 percent of the poverty line.

Moreover, many categories of people ineligible for cash welfare, such as childless couples and single people, could get food stamps.

The result of food-stamp enactment was to even out differences in total welfare received by people in different states. Thus Mississippi in 1980 paid $120 a month in cash welfare to a family of four with no other income, less than a quarter of the cash payment available in New York.

But after food stamps were figured in, the Mississippi family ended up with $3,948, and the New York family with $7,836. The final Mississippi level was half New York's. In general, welfare benefits are low and food-stamp benefits high in southern states.

Even including food stamps, studies by the Department of Health and Human Services show that the buying power of the average welfare recipient nationwide declined 8 percent from 1974 to 1979 because the states lagged so much in keeping the AFDC cash payments up with inflation.

Greenstein predicted that eventually the states would simply "cash out" the stamps (i.e., abandon the use of stamps and simply increase welfare payments slightly to make up for it); that they would not make their programs available to childless couples and single people, who now qualify for stamps but not for AFDC, and that they would not "index" the new programs to the inflation rate, as stamps are now.