John Baize, Washington representative of the American Soybean Association, is delighted that President Carter has ordered a billion-dollar increase in price supports for grain farmers. But he cannot help chuckling about it.

"We'd asked and asked for the increases, and the administration kept saying it couldn't be done," Baize said. "Then all of a sudden on Friday the Senate passed the price supporters, and we get a call on Saturday from the White House to come in Monday for the announcement."

It is called incumbency politics and it's a weapon that Carter can be counted on to use effectively if the performance on grain supports is any evidence.

Carter grabbed the headlines Monday with a dramatic announcement designed to show how much he could do for the nation's farmers -- an influential voting block. All he had to do was agree to the inevitable, for the price-support bill was speeding through Congress with unstoppable momentum.

Republicans such as Sen. Bob Dole of Kansas were taking credit for the bipartisan price-support amendment that was attached Friday as a rider to the federal child nutrition program.

"But the president pulled the rug out from under them by doing it administratively," said a National Farmers Union official, "Politically, they were scooped."

While farmers are a dwindling breed, agricultural issues are important in large areas of Texas, Illinois, Iowa and Minnesota -- key states where the farm vote could mean the difference between winning and losing in November.

For months, farm groups have been pushing for an increase in price supports to offset the effect of the grain embargo imposed against the Soviet Union in January after its invasion of Afghanistan. Finally, over the weekend, Baize said, Carter aides "sawa situation where they had better act or write off the farm vote."

It is not the first time that the Carter administration has acted suddenly to placate farmers: In April, just before the Wisconsin primary, administration budget-cutters dropped from their proposed 1981 budget a plan to cut dairy price supports.

Monday's action goes a long way toward soothing the feelings of farmers who resent having to bear the economic burden of the Soviet sanctions. But the move may be largely symbolic, because the drought and other factors have caused farm prices to rise. Higher prices mean that farmers will be able to pay back most of the $1 billion increase in price-support loans.

Put in political terms, Baize said that "at this point [GOP] presidential nominee Ronald Reagan has the majority of the farm vote. But come election time, what with Monday's action and [an expected] announcement in October for 1981 crop price supports, it could be a tossup."

Mark Schmidt, communications director for the Illinois Republican Committee, said yesterday that the new price supports will give the president "a little bolster in the short run, but in the long run it won't help that much.

"It's too little, too late. A lot of Illinois farmers voted for Carter in '76. Now with the embargo, the feel cheated. A lot is going to be decided based on how the economy is doing in November."

In Texas, Democratic state chairman Billy Goldberg said farm issues, including rising costs and the grain embargo, resulted in several delegates to next month's national convention in New York being elected as uncommitted. t

Carter called Monday's move "remedial action" for the grain embargo and "a step in the right direction."

While some analysts suggest that the drought, which has brought a speculative surge in commodity prices, could help Carter in the farm belt, Johnson and other farm leaders doubt that it will be a factor.

"If the drought [reduces the harvest] so the price goes up, some families might benefit," Johnson said. "But a hell of a lot wouldn't" because high prices do not help people whose crops have lost, he said.

Farm income is expected to drop significantly this year from 1979 levels because of high interest rates and the rising cost of fuel, fertilizer and equipment.

Price supports amount to a floor for grain prices -- a guaranteed government price -- $3 a bushel for wheat, up from $2.50; $2.25 for corn, up from $2.10 and $5.02 for soybeans, up from $4.50.

The government advances money to the farmers and if market prices are higher than support prices, as they are likely to be, the farmers repay the loans.