President Reagan attempted to give farm-state Republican lawmakers ammunition for their campaigns yesterday with an announcement that the Agriculture Department would allow the Soviet Union to purchase another 10 million metric tons of American grain starting Oct. 1.

Reagan had said he would allow the Soviets to buy more grain, but several GOP incumbents pressed the White House for a formal announcement to fend off attacks from Democratic challengers, officials said.

The president complied by making the announcement at a Cabinet Room meeting with lawmakers, including Sen. Roger W. Jepsen (R-Iowa) and Sen. Charles H. Percy (R-Ill.), who face reelection challenges.

Reagan announced the "ceiling for the second year" of the five-year Soviet grain agreement had been lifted from 12 million metric tons to 22 million metric tons of corn and/or wheat.

The second year begins Oct. 1.

A metric ton is 2,204.62 pounds.

Agriculture Secretary John R. Block said that the Soviets have not expressed an interest in buying that much grain but that the Reagan administration wanted to leave the door open.

"I've said many times our philosophy is against the unfair and wrongheaded policies of grain embargoes, and we're going to continue to do everything we can to strengthen markets for America's farmers," Reagan told the lawmakers.

The White House said the Soviets have purchased 14 million metric tons of corn, including 6.6 million in the first year of the agreement, and more than 9 million of wheat, including 7.8 million in the first year, since the long-term agreement with the Soviets was signed in August 1983.

During the first year, the Reagan administration also decided to extend the ceiling by 10 million metric tons.

Agriculture Department analysts have estimated that this year's Soviet grain crop will be about 180 million tons.