There's trouble brewing for the Carter White House in the Farm Belt. Farmer income is now headed toward its lowest levels since the late 1960s, and, if farmers vote their pocketbooks, many of the key agricutural states probably will end up in the Republican column in November.

That's where most of them were in 1976, and that's where the conventional political wisdom says they belong. Farmers are cautious and conservative, and so they vote Republican. But the conventional wisdom forgets the substantial populist sentiment among farmers -- particularly in the South and upper Midwest -- and the increasing urbanization that has torn many traditional farm states from their Republican roots.

What's also forgotten is just how close the 1976 election was. Jimmy Carter won only 50.1 percent of the popular vote, and the victory of margin in many states amounted only to 2 percent of 3 percent of the vote. Consider the following list of key Midwest farm states, showing the winner's percentage of the vote:

Illinois, 50.1% (R); Indiana, 53.3 (R); Iowa, 49.5 (R); Kansas, 52.5 (R); Michigan, 51.8 (R); Minnesota, 54.9; Missouri, 51.1 (D); Nebraska, 59.2 (R); North Dakota, 51.6 (R); Ohio, 48.9 (D); South Dakota, 50.4 (R); and Wisconsin, 49.4 (D).

On this evidence, virtually all these states -- with the possible exception of Nebraska -- ought to be up for grabs this fall. but the prospective drop in farm income could make any real contest illusory.

According to the Agriculture Department's latest estimates, farmer net income in 1980 could drop by between one-fifth and one-third from 1979's total of $33 billion. Translated into 1967 "constant" dollars, the department estimate would mean lower income than in the unhappy years of 1976 and 1977, which brought the tractorcade to Washtington and lower than in the late 1960s. The comparison with the 1960s. The comparison with the 1960s is misleading because the number of farms has dropped, but the dip since 1977 has been slight.

To say that these states vote only on the basis of farm issues is, of course, preposterous. Even in South Dakota, the farm population is only about one-fourth of the total; nationally, it's less than 5 percent. But the economic and political impact of farmers is geater than the numbers suggest. In major grain states such as Kansas and Iowa, farmers' prosperity affects the rest of the local economy, creating demand for farm equipment sales and consumer goods. In industrial states, the farm equipment sales and consumer goods. In industrial states, the farm vote can prove decisive.

What is distinctive about the current slump is that is seems to have hit virtually all sectors of farming. Usually, trends in farm prices tend to favor one farm group over another. A decline in grain prices, for example, tends to help meat producers because grain is one of their major operating costs. This year, both groups appear to be hurting.

Traditional Democratic states may be hardest hit. Minnesota farmers feel especially aggrieved, according to Luther Pickrel, a professor at the University of Minnesota's extension school.

Last summer, a strike of grain handlers at Duluth crippled shipments. Now, these same famers -- relatively distant from markets -- feel that high transportation costs cut into their grain prices and add to the costs of their suppliers, says Pickrel. And, he adds, they're blaming the administration's partial embargo of grain shipments to the Soviet Union for their plight.

This may not be fair. The administration went to extraordinary lengths to insulate farmers by buying grain on the open market and by taking extra amounts into longterm storage under government loan. In addition, there's almost certainly been a game of musical chairs among importers and exporters. As the Soviets attempt to buy from other exporters, some demand from Thir World countries has shifted to the United States. Corn and wheat prices are still at last year's levels, and, despite the embargo, the Agriculture Department projects record food exports of $38 billion, up by nearly one-fifth from 1979.

Whether exports and prices would have been still higher without the embargo is anyone's guess. But farmers certainly are entitled to think so -- and probably will.

Their basic problems lie elsewhere and aren't entirely of the administration's making. Grain farmers have suffered from stiff rises in fuel and fertilizer costs, both reflecting higher oil prices, and a simultaneous jump in interest rates. Costlier money also has hurt meat producers, who need large amounts for feed and the purchase of young livestock. Together, these factors account for more than half of the $10 billion to $16 billion increase in farm operating costs expected by the Agriculture Department this year.

At the same time, hog prices have collapsed. Responding to higher meat prices and lower grain prices in 1977 and 1978, producers expanded output significantly. But rising production and a limping economy now have resulted in a price decline of about one-third from year-earlier levels. That's not mainly the administration's fault. Nor is the slump in soybean prices (down about one-sixth this year), which relects large harvests both here and in other producing countries such as Brazil.

But the White House is likely to get blamed anyway. The irony is that the administration -- partly under pressure from Congress and partly of its own volition -- has been relatively kind to farmers in setting support prices and loan rates for grain producers. It's arguable that a Republican administration would have been a lot less forthcoming.

A final frustration involves consumer food prices. All the farmers' unhappiness might be worth the trouble if a dramatic decline in retail food costs had broken inflation's back. It hasn't. About 60 percent of food prices represent marketing, packaging and transportation expenses, which tend to rise as labor and energy costs increases. Consequently, food prices have climbed 7 percent in the past year, even though meat prices -- influenced by large pork and poultry supplies -- haven't increased at all.

A lot, of course, can happen between now and November -- to crop conditions, farm prices and polictical fortunes. Meanwhile, Carter administration officials undoubtedly regard the current situation as highly unfair. It probably is, but, as their boss once said, life is unfair. It's an old axiom of politics that the powers that be get blamed for the problems that be.