All across the winter-blitzed Midwest, the clouds of a massive agricultural economic crisis are quickly taking shape, threatening to throw thousands of farmers out of business and send country banks and businesses toppling.

The Great Farm Shakeout that many have predicted since 1981 appears to be at hand, with no clear sense of how far the ripples of failure will reach into the national economy or of how rural America will be reshaped.

The fears and frustrations of farmers, public officials and businessmen, whose cries for emergency federal credit assistance show little sign of being heeded, are compounded by a feeling that Washington isn't listening and doesn't care.

On Friday, for example, as Iowa Gov. Terry Branstad was saying that he saw a bit more sympathy developing in the Reagan administration, newspapers were quoting Agriculture Secretary John R. Block as saying, "The circuits are overloaded [and] this is a time to tough it out."

Officials and economists in Iowa and Nebraska say the next two months are critical to agriculture, with 1984 notes coming due and farmers going to lenders seeking credit to put in their new crops. Thousands will not get it, leaving them in foreclosure or bankruptcy, and leaving their heavily leveraged farm-supply creditors in the lurch.

More ominously, Iowa State University economist Neil Harl said last week, at least 40 percent of the farmers in the North Central region -- Minnesota to Missouri, Ohio to Kansas -- have such burdensome debt that they are "on a conveyor belt, headed down . . . headed toward insolvency" in the next several years.

Those farmers are in jeopardy, Harl explained, because their debt loans are growing rapidly while the value of their assets, such as land and tractors, is dropping steadily. Only a surge in farm prices or a sharp interest-rate drop will help them, and few economists foresee either.

Harl added, "The next 70 days are vital because the decisions are going to be made on who gets credit. We are going to go through a terribly stressful period . . . . We must use this time creatively. We know times are stern in Washington, but we do believe we have to have some federal assistance to avoid the devastation. If, at least, we don't keep interest payments current, then lenders are going to go down the tubes also."

In Washington, farm-state legislators are pressuring the administration to quickly come up with additional credit and loan guarantees to help the battered farm sector through the growing storm. Senate Majority Leader Robert J. Dole (R-Kan.) and others want a presidential task force to tackle the problem.

Whatever is done must be done swiftly, officials here say. In Iowa, the nation's No. 2 farm-income state, economists say that as many as 17,000 of the 113,000 farmers will go under by late spring as their applications for 1985 operating-money are rejected. More than 130 banks in Iowa are listed by the state as having "more than normal problems" and 48 of them are on a Federal Deposit Insurance Corp. "problem list."

In neighboring Nebraska, where about two of every five jobs are related to agriculture and where six country banks and two farm- credit banks have failed since La- bor Day, at least 10 percent of the 63,000 farmers are expected to shut down for lack of operating credit.

"The problem is at the crisis stage," Nebraska banking commissioner Roger Beverage said. "There sure as hell is no economic recovery in Nebraska and it spills into all we do on Main Street . . . It is a parade of horribles. The people in Washington have to realize this is not a joke. This is a depression."

The story is similar all around the Midwest, the fertile bread basket that produces the bulk of the wheat, corn and soybeans that feed the nation and bring in billions of export dollars.

The reasons for the crunch of 1985 are many, but they can be boiled down this way: Low farm prices, continuing high interest rates, plummeting land and machinery values that rose beyond worth in the inflationary 1970s, a strong dollar that has depressed exports and overproduction of basic goods.

A key culprit is the interest rate that has stayed high as inflation cooled. Illinois farmers, for instance, have paid more than $1 billion in interest in each of the last two years, compared with $430 million in 1977. Nebraska farmers, with the sixth largest debt load in the country, have paid more than $1 billion in interest in each of the last three years, compared with $50 million in 1960.

"High real interest rates and rising debt loads have combined in the 1980s to sap the strength of the Illinois farm economy," said John White Jr., president of the Illinois Farm Bureau. "Lower interest rates could help bring profit back to the farms."

But, added Iowa Gov. Branstad, "It is a lot more than just the farmers. We are talking about businesses on Main Street. President Reagan's debt-restructuring program hasn't worked and it appears that the Office of Management and Budget has blocked the key elements to make it work . . . The downward spiral is speeding up and it has deteriorated even further in the last few months."

Both Branstad and Harl, in separate interviews, described their alarm after meeting last week with OMB Director David A. Stockman. Both said that Stockman lectured them and Block about the administration's determination not to provide a bailout for a localized problem.

These and other Washington reports have ignited bipartisan fears that the administration has decided to pull the plug on an industry that it believes has more producers and more production capacity than it needs.

In Nebraska last week, farmer-rancher Tom Vickers, also a state senator, said, "The Nebraska economy has been on a downhill slide for a number of years. We're about to see a disaster this state hasn't seen since the 1930s. Farmers and bankers are going to go under, and what happens here affects the whole nation."

The state legislature quickly passed a Vickers resolution calling on the legislatures of nine nearby farm states to send a delegation to Washington next month to try once more to deliver the call for help. Several states already have joined Vickers' campaign.

Bruce Johnson, an economist at the University of Nebraska, added that "literally half of the state's agricultural wealth has been lost since 1981. This ripples up through the financial institutions. When you aggregate the loss, how many banks does it take to affect national institutions? Crisis is not too strong a word to use for what is happening out here."

The numbers generate strong fears. William Nichol, the Republican speaker of the state legislature, said, "I am frightened and I never have been frightened before -- and I lived through the Great Depression. Nothing is of any value and there doesn't seem to be anything to change it. I see nothing but deterioration in the months ahead."

"It's scary, sure it is," added GOP state Sen. Rod Johnson, 27, a farmer who chairs the legislative agriculture committee, "and we're not sure the federal government understands what is happening. I think Ronald Reagan owes the farmers something."

Adding to the current mix, many rural banks and farms of the farmer-run farm credit system have started to tighten the screws on borrowers who are not delinquent on loans, insisting that they sell land and equipment on a market where such commodities are going at fire-sale prices -- if a buyer can be found.

"Some banks are notorious on this," said the Rev. David Ostendorf, Midwest director of Rural America. "People are then forced to go to the Farmers Home Administration or elsewhere to seek credit -- and those systems are struggling already. People are being dumped every day, and it's outrageous."

As the tension and the frustration build, farmers have begun to band together to draw attention to their plight. A St. Paul, Minn., rally last week drew 10,000 protesters. Another in Sioux City attracted more than 4,000. Farmers fill the state capitol building here daily, lobbying their legislators, and they're planning a major rally for Des Moines next month.

The Iowa Farm Unity Coalition, a family-farm-advocacy organization that has operated an informational telephone hot-line for farmers since late 1982, also is stepping up its efforts to publicize the crisis. This week, for example, it will attempt to stop another forced sale of farm machinery -- a practice that has become increasingly common throughout the region.

The coalition also is promoting a campaign to force Branstad to declare a moratorium on loan foreclosures. The Iowa legislature has passed a resolution that could lead to declaration by the governor of an economic emergency and possibly a moratorium. Branstad has appointed a committee to study the idea.

"People out here have begun to realize that the whole house will topple if we do not take steps very soon," said Ostendorf, who also is a coalition official. "The question is whether we can act fast enough and get a response from Washington. It will mean a massive amount of money for debt restructuring and it will mean a new federal-state relationship in debt handling."

"We wouldn't have been in this situation if public officials had taken the appropriate steps three years ago. It could have been avoided if the politicians had listened," Ostendorf said. "Now they're reaching for the panic button and we're into crisis management once again."