From the Red River Valley to the Carolina tobacco fields, through the heartland of central Illinois and across the Kansas wheatfields, normally conservative farm districts are key battlegrounds in the struggle for control of Congress.

The farm vote, in 1980 one of the most solid blocs for Ronald Reagan, is now a testing ground for the president's policies as corn, wheat, soybeans and sugar beets sell for less than the cost of production, farm income falls to 1933 levels, International Harvester teeters on the brink of bankruptcy and farm auctions are held in the midst of the harvest.

The near-depression in rural America is the reason fully half ofTHE FARM VOTE the 85 competitive House races for the Nov. 2 election are in districts where agriculture plays a major, if not dominant, economic role. For President Reagan, these districts are essential for continued control of Congress.

Thus, Reagan is making a determined last-minute bid to keep the farm vote Republican. Last week he offered to sell the Soviet Union 23 million tons of grain. Yesterday, he began a campaign swing into the heartland of the Midwest, starting in Peoria and moving on to Nebraska.

In these farm districts, politicians and polls agree that Reagan's economic policies are, to an unusual degree, superseding local issues. Many farmers have been forced into a unique political and economic corner, as their land and capital, often worth well over $1 million, are eaten away by mounting debts and crops that don't pay off.

Signs of discontent are surfacing:

In Westbrook, south of here, farmers adopted a Depression tactic, the "penny auction," to halt a foreclosure. Wearing red bandannas, farmers moved through the crowd warning potential buyers of the farm and equipment to keep bids to no more than a "dime or a dollar." The auction was canceled.

In Detroit Falls, to the north, farmers marched into the Becker County Farmers Home Administration and forced officials to defer for a year collection of a $128,000 loan from Joel and Sandra Nelmark, dairy farmers whoseTHE FARM VOTE loan repayments were amounting to 60 percent of their annual income.

In an almost pleading vein, Rep. Arlan Stangeland (R-Minn.) told a group of adult students at Detroit Falls Vocational Tech, many off the farm seeking training for other work: "There is the hope that with economic recovery, interest rates coming down, there will be jobs, there will be a tremendous boom. . . . There is definitely the start of a turnaround."

Gene Westrom, his opponent, counters: "These guys [Republicans] were running around two years ago with all the answers, and now they don't want to accept responsibility. I don't think the public is going to stand for it. This is going to be a referendum on Reaganomics."

A Washington Post-ABC News poll of 1,680 voters in 19 rural congressional races supports the view that their outcome will be determined by the voters' assessment of Reagan's economic program.

Despite the fact that Republican incumbents outnumber Democrats in these districts by a ratio of 8 to 4 (six others are open seats and one has two incumbents facing each other in a redrawn district), support for Democratic congressional candidates exceeds that for Republicans by 44 percent to 40 percent.

Perhaps more significant is the finding that confidence or distrust in Reaganomics is more important in determining voter intention than military spending, inflation, concern for the poor or assessment of the two political parties' programs.

This was true of Democrats, Republicans and independents. Among Republicans who say they are "very confident that Reagan's economic program will improve the economy," 95 percent say they will vote for the GOP congressional candidate in their district. Among those who are "not confident at all," Republican support for GOP congressional candidates is halved to 47 percent, while GOP support for the Democratic candidate grows to 29 percent.

Similarly, 62 percent of Democrats with high confidence in Reagan's economic policy will vote for the Democratic candidate, and 23 percent will vote Republican. About 83 percent of Democrats with no confidence in Reaganomics will vote for the Democratic congressional candidate, while only 7 percent would support the Republican.

Among independents, the largest number of undecided voters, firm believers in Reagan's program will vote Republican candidates by 61 to 29, and those most disenchanted will vote Democratic by 58 to 26.

In 1980, Democratic candidates in agricultural districts were crippled by Jimmy Carter's presence on top of the ticket because of his embargo on grain shipments to Russia after the Afghanistan invasion.

Reagan lifted the embargo, but retaliated against suppression of the Solidarity union in Poland by banning sales of U.S. equipment for the Soviet pipeline. Grain dealers at the Chicago Board of Trade believe the Soviet Union will, in turn, refuse to buy the full proffered 23 million tons of grain, and will deal U.S. farmers a blow by buying as much as possible from other nations such as Canada and France.