With most preliminaries out of the way, the House and Senate Agriculture committees are zeroing in on their Mission Improbable for 1985 -- writing a farm bill that avoids a presidential veto, cuts federal spending and keeps thousands of economically endangered farmers in business.

House subcommittees began fine-tuning sections of a bill this week, with Chairman E (Kika) de la Garza (D-Tex.) shooting for the full committee to complete work on the bill by May 15. The Senate is expected to begin work on its version shortly thereafter.

No one expects a bill to be written easily or quickly, however, with U.S. farmers under the most intense economic stress since the Depression and demands rising steadily for more feder al help and a go-slow approach to change.

Stagnant prices, overproduction of most commodities, slumping exports, high interest rates and falling land values have set off an epidemic of farm failures and foreclosures that has put farm-state legislators facing reelection next year in a cold sweat.

But several points are clear as the committees get down to business:

* President Reagan's proposals to reduce farm-support costs radically and reorient farmers toward a "free market" economic policy formally died on the day that Agriculture Secretary John R. Block told the Senate committee that exports and farm income would decline sharply during the first two years of a Reagan bill. (Farm spending has totaled more than $50 billion since Reagan took office.)

* Most committee members agree that farm programs are not working and must be reshaped, although few have rallied around the welter of proposals served up and most concede that many of agriculture's problems -- taxation, monetary policy, a strong dollar -- are beyond the reach of a farm bill.

* While the Reagan bill may be dead, the budget-cutting and market-oriented ideas that spawned it are not. Both hang heavily over the committees as they grapple to come up with a program that meets those goals, while targeting federal assistance to the middle-sized farmers who need help the most.

"There are some darned good answers being offered," said Rep. Thomas A. Daschle (D-S.D.), a House committee member, "but there is a 1,000-pound gorilla at the end of the road who says he'll put a v-e-t-o on anything that comes down that doesn't satisfy him. And that has stifled some of the creative thinking."

"But there is a recognition that we must have some income maintenance, we must have a farm credit section and a strong conservation title in any bill we come up with," Daschle said.

Sen. Rudy Boschwitz (R-Minn.), coauthor with Sen. David L. Boren (D-Okla.) of an unusual "full production" farm bill proposal, said the urgency of reshaping farm policy extends beyond farmers.

"We must protect farming income and direct support at the family-farming structure. And we must make the whole rural infrastructure system work. What I call the infrastructure -- the banks, the suppliers, the businesses, Main Street -- is as fragile as the farmer himself," Boschwitz said.

Another Republican who has parted ways with the administration, Rep. Pat Roberts (Kan.), said, "I think there is more of a consensus than people realize. There has been more than 18 months of intensive study, and the problem is pretty well defined. Reducing that to one word, the problem is price."

Roberts echoed a concern that runs deeply through both committees -- the farm economy. "We know how grim the economic situation is, and we know we have to hang together. I find myself in a minority in my party taking a look at the factors that impact price at the country elevator. Even being conservative, there will be a helluva shakeout. Our hope now is to avert a blowout."

Roberts, Daschle, Boschwitz and Boren, among others, are pushing ideas that would encourage export competition by lowering the support loan rates that tend to put a floor under U.S. farm prices, while continuing direct income subsidies to the most hard-pressed farmers. Senate Agriculture Chairman Jesse Helms (R-N.C.) has introduced a variation on those ideas that will be the working "vehicle" for the Senate markup.

A recurring theme in most of the legislative proposals relates to exports, one of the most worrisome elements in the farm picture.

Roughly 40 percent of U.S. farm production is exported, but recession and increased competition have sent U.S. sales into a tailspin since their $43 billion peak in 1981. The Agriculture Department estimates that sales will total about $34 billion this year.

Critics from both parties argue that current programs, with price floors above market levels, contribute to the decline. Competitor countries, some with heavy farm subsidies, undercut U.S. prices and produce more when the U.S. programs call for production cuts. And, these critics say, the higher support levels here encourage farmers to grow more and then forfeit their crop-loan collateral to the government.

"We no longer are going to be the chumps who cut back production while our competitors overseas expand," Boren said. "We are going to win back more bargaining leverage."

Another element new in the mix is that the debate includes more than farmers and legislators. Grain companies, agribusiness, and food-processing and consumer interest groups have joined in pressuring Congress for changes in basic farm law.

One group, Public Voice for Food and Health Policy, which seeks major changes in the dairy program that has cost the government more than $6.1 billion since 1981, brought members of local consumer groups here in January for training on farm-bill lobbying.

Executive Director Ellen Haas said Public Voice will concentrate on the dairy portion of the farm bill, pushing for lower price supports that would translate into lower retail costs but encourage a steady supply of milk products. A key issue in the dairy section as well as others, Haas said, is "fairness."

"There are more new players in the dairy fight this time -- food stores, individual processors," she said. "Consumers haven't shared in the large production gains that have occurred in recent years. Assuring supply . . . is not enough in today's world."