In six short months, Jimmy Carter has transformed an inadvertent political asset in the traditionally Republican Midwest into a liability that is corroding his political base throughout the Wheat Belt.

The inadvertent asset was the previous administration's embargo on shipments of wheat to the Soviet Union. Regarded by wheat farmers as a betrayal, the ambargo helped send soaring wheat prices into a tailspin, wheat farmers into recession and Gerald Ford into political disgrace.

Even with Sen. Robert Dole of Kansas as Ford's running mate, the Carter-Mondale ticket would have taken Kansas with a switch of only 36,000 votes (and Oklahoma, a major wheat state, with a switch of 7,000 votes). Deserting Ford in anger. Wheat Belt farmers voted in surprisingly heavy numbers for the Carter-Mondale ticket.

Now disillusion has set in with severity as the Carter-Mondale campaign promises of high price supports have turned to ashes. "We got the shaft," Ralph Krehbiel, a Hutchinson wheat farmer who is also Reno County commissioner, told us. "Carter said one thing, then he did another."

Thirty miles southwest of here, in the hamlet of Lerado, Wesley Nunemaker, a lifelong Democrat and one of Reno County's most successful wheat farmers, recalled a springtime lobbying trip to Washington. "I told them in the Agriculture Department that we wanted a recall election out here," he said. "If we had one, Carter would get a lot less votes."

The reason is simple: a series of broken promises by Carter and Vice President Mondale that many wheat farmers suspect resulted from a deliberate effort to cash in on the misfortunes of Jerry Ford with no real intention of following through. Last August, Carter made a speech at the Iowa fairgrounds that Kansas wheat farmers recite from memory. He pledged a farm program with support prices "at least to equal production costs, and you can count on that."

Then, one month later in Wichita, Kan., Mondale gilded that "pledge" in a radio interview with KRFM in which he said that the "production costs" of Kansas wheat farmers for a bushel of wheat "are at least $3" - a statement that, juxtaposed with Carter's, could only mean one thing: a $3-per-bushel price support level in a Carter administration.

In fact, in the Carter-Mondale farm bill that went to Congress on Jan. 17 - three days before the inauguration - the proposed level for the support price for wheat (at which the government will buy wheat to prevent prices from falling to crisis levels) was not $3 but the same old $2.25 (actually $1.96 after subtraction of transportation and storage charges) for which Carter and Mondale had attacked Ford throughout the campaign.

Even more damning in the eyes of Kansas wheat farmers was the fact that the Carter farm bill initially set the "target price" of wheat in the 1977 growing year - a key price in computing other farm subsidies - at a mere $2.47 a bushel. That was far below most estimates of the cost of growing wheat, Earl Hayes, president of the Kansas Association of Wheat Producers, and most other wheat growers we talked to, agreed that the average cost of producing a bushel of wheat is at least $3 - the figure Mondale used in Wichita.

Max Deterding, a country banker in Turon who, with every other bank in Kansas, is now refinancing delinquent farm loans with wheat farmers whose backs are against the wall, explained the farmers' anger. "I took all those Carter promises with a grain of salt," he said. "But farmers out here believed them. They are absolutely certain that Jimmy Carter cheated on them, and I don't think they'll forget it anytime soon."

Deterding is not foreclosing any of his loans today, but he is refinancing many of them. So is the local grain coop and the commercial elevator whose storage loans aren't being paid.

The reason they can't collect is that inflation in everything but farm prices is destroying the family farm. In the past four years, diesel fuel has gone from 13 to 39 cents a gallon, plow blades from $30 to $78 for a package of five, cultivator shovels from $1.60 to $3.10. In that same period, the support price of wheat has gone from $1.25 to an effective $1.98 a bushel.

Clearly, President Carter's concern about balancing the budget exceeds his concerns about unfulfilled pledges to wheat farmers. Savings in farm subsidies might be good for the country - but wheat farmers see it differently, which goes far to explain the long and acrimonious battle over the new farm bill in Congress.

However that turns out, the President has already struck out here in the Wheat Belt, losing a constituency that wanted to join with him in future political battles. The loss of future support goes hand-in-hand with the threatened defection of the blacks and other disappointed voting blocs successfully courted by Carter last year.