For months, it has been building steam in the wings as one of Washington's most beguiling political and social dramas, and now the debate on the fate of American farmers and of the government programs that support them moves to center stage.

The House plans to start voting today on a 1985 farm bill, beginning with controversial amendments that would pay dairy farmers not to produce and would continue to prop up sugar producers. The Senate is scheduled to begin debate on its bill Oct. 15.

Through painful preliminaries in the House and Senate Agriculture committees, this debate has been as simple and complex as a legislative issue can be -- as simple as the price of a loaf of bread, as complex as the restructuring of America's great farm-production machine.

American farmers grow more food than the country can use or sell abroad. The questions are whether and how the public should pay to keep the food machine running, keep farmers on the land and, in a sense, sustain the way of life symbolized by agriculture.

The debate is punctuated by crushing economic problems on the farm. Bumper crops, combined with falling prices, collapsing sales abroad and a severe credit crunch, add an air of urgency.

The bottom line is money -- how much the government spends to fuel the machine, the consumer pays for food, the farmer receives for goods, the processor spends on raw ingredients. The debate, cutting through the rhetoric, is about how these costs are apportioned.

The Reagan administration contends that federal support must end now, requiring farmers to adjust to lower prices and diminished markets. The other side, which includes sizable numbers of Republicans, argues that the government put farmers in their present fix and must help extract them with compassion and generosity.

Two issues expected to surface early in House floor skirmishing illustrate the points:

*Dairy -- The committee is offering a complex plan that pays farmers not to produce, to reduce the milk, butter and cheese surplus that has cost the government more than $6 billion since 1981.

Supporters agree that milk prices would not decrease but that the farmers' pain of cutting output would be eased. Opponents say an easier, quicker way is to cut federal supports sharply, regardless of whether it means bankruptcy for some farmers.

*Sugar -- The committee wants to continue a program that props domestic sugar producers and keeps prices high through restrictions on less expensive imports.

Critics who want to scuttle the program say it threatens jobs in sugar refineries and candy factories and on the docks, while punishing Latin American allies who rely on sugar as a major source of income.

These and other farm-support issues promise heated argument in House and Senate. But the debate is driven by perhaps more immediate forces -- enormous crop surpluses pushing prices to low levels, the most farm foreclosures and country-bank failures since the Great Depression, collapsing export sales, high interest rates and falling land values that imperil the stability of thousands of farmers.

Wrangling over these issues in the Agriculture committees has been intense all year, with farm-state legislators torn between administration insistence that federal program spending be cut and their own perceptions that rural America can be kept strong only with greater federal support.

In the background is farm credit deterioration. The farmer-owned Farm Credit System holds about one-third of the $214 billion U.S. agricultural debt, and private farm lenders intend to seek federal financial aid to cover increasing amounts of bad farm debt.

Also influencing the debate is raw politics. With 22 Senate Republicans' seats to be contested next year, farm-state GOP incumbents have been extraordinarily leery of backing administration budget cuts.

The most striking example is Sen. Mark Andrews (R-N.D.), who, facing a reelection campaign, has consistently lined up with committee Democrats in insisting that federal farm spending not be cut.

Nevertheless, many congressional observers say a farm bill can do relatively little to unsnarl a U.S. agricultural machine that produces more than it can sell. They say monetary policy and the world economy, which in good times buys at least one-third of American farm output, have much to do with U.S. farm solvency.

As Sen. John Melcher (D-Mont.), a leader of Senate forces arguing strongly against administration efforts to cut federal supports, says:

"A group of us started from the premise that the situation for price correction -- improved farm income -- is beyond the scope of the bill, except to the extent that we could improve trade and food-aid provisions utilizing the surplus commodities that we have . . . . We have come part of the way on both items."

He and his allies, supported by Andrews, dominated Senate committee debate and, as the panel approved a farm bill last week, rolled over Chairman Jesse Helms (R-N.C.) and Majority Leader Robert J. Dole (R-Kan.) to produce a measure at least $9 billion over budget.

The administration calls the bill unacceptable.

"The brutal truth is that the times are so serious with low farm prices and the pressure of getting credit that there isn't any time for politics," Melcher said yesterday. "What we are trying to do is a minimum that has to be done, and it goes way beyond the farm economy into the national economy . . . . It is too serious, too critical to pay any attention to the administration."

Across Capitol Hill, Rep. Edward R. Madigan (R-Ill.), like many Republicans, has distanced himself from the administration's hard line on farm policy.

"There are two categories of people who are involved and are about to become involved," he said. "The first is people who are trying to develop a workable farm bill . . . to stabilize the farm economy. The second is people who think that is not possible to do and who are looking for votes to protect their rear ends at home."

Somewhere between those lines lies a 1985 farm bill.