For all of that political clout they are purported to have on Capitol Hill, President Reagan and his Agriculture secretary, John R. Block, are having a miserable time trying to use it on farm-state legislators these days.

The scene features such recent attractions as Sen. Roger W. Jepsen (R-Iowa) saying the president has slapped U.S. farmers in the face and a defiant House threatening successfully to crush a Reagan veto.

Most insiders say they think that Block, after a shaky start, has developed into a skilled persuader. But he continues to have a hard time getting his way with Congress because many legislators think Office of Management and Budget Director David A. Stockman, rather than Block, calls the shots at the Agriculture Department.

Partly because of that, Block has been unable to get Congress to go along with his effort to freeze the amount of direct subsidies the government will pay to farmers next year. Some legislators think a freeze would throw hard-pressed farmers to the wolves.

Congress threw another curve in August, delaying a new dairy fee that was intended to get farmers to slow milk production. The USDA now spends more than $2 billion a year buying the surplus. And Reagan, long eager to cut dairy costs, vetoed that one.

But when he vetoed another bill, pushed by Rep. Berkley W. Bedell (D-Iowa), the fat was in the fire. Bedell and other farm-state members, abetted by the National Fertilizer Solutions Association and the wheat growers, made it clear that the veto would be overridden.

The bill, requiring Block to announce next year's farm programs earlier than he wanted, had been pushed by the fertilizer dealers, who argued that it would help farmers plan for 1984 and help fertilizer sales.

Faced with the threat of a veto override, and aware that a sensitive election is just around the corner, the administration capitulated last week and agreed to a compromise if Bedell would back off. The compromise gives Block a little more elbow room than the vetoed bill did.

"We opposed it because it limited the secretary so much," a USDA official said last week. "I guarantee you nobody ever thought that bill would pass. But then Bedell and Rep. Pat Roberts R-Kan. pushed it, the fertilizer people got involved and it snowballed . . . . People just got caught up in the emotional claptrap."***

HIGH AND DRY . . . Last year, when the eyes of Texas were focused on a torrid gubernatorial race between incumbent Republican William Clements and Democrat Mark White, the GOP-controlled USDA found a way to channel about $200 million in low-yield disaster payments to Texas cotton farmers hit by bad weather.

Politicians who understand these things saw the USDA gesture as an attempt to bolster Clements' stock in a pivotal political state. Iowa, in contrast, where farmers were hit by a similar disaster, couldn't get any help. But there was no hot election going on there.

Now, White occupies the governor's chair in Austin and another Democrat, the very partisan and very outspoken Jim Hightower, is the state's agriculture commissioner and chairman of a Democratic National Commitee farm policy committee. They can't get the time of day from the USDA.

For weeks they have pleaded for special help for ranchers in 26 far west Texas counties, where 20 months of drought have decimated cattle, sheep and goat herds by virtually eliminating grazing possibilities. But the USDA has rejected their requests for surplus federal grain to feed the stock.

White and Hightower argue that the traditional loan assistance the USDA is willing to provide won't help ranchers much. White invited Block to visit Texas and see for himself, but the secretary said he couldn't make it. A request for him to send some aides in his place has drawn no response.