Before it is even off the ground, the government's new soil conservation reserve is under attack on Capitol Hill by the unlikeliest of foes -- Rep. Jamie L. Whitten (D-Miss.), a champion of other conservation programs.

Whitten, chairman of the House Appropriations Committee, has proposed major changes in the conservation reserve as part of an urgent catchall appropriations bill that the panel is to consider today.

Conservationists and the Agriculture Department were attempting yesterday to mobilize opposition to Whitten's proposals, but conceded they faced an uphill battle because the chairman's supplemental bill contains funding for other politically popular programs.

"It's got all the trinkets imaginable and it will appeal to a broad political constituency," a USDA official said. "But we're opposing the entire bill and we have been assured that it will have a rough way to go in the Senate if it gets through the House."

The reserve was created by last year's farm bill to pay farmers to remove as much as 40 million acres of highly erodible land from production over the next five years. The idea was to save vulnerable soil, reduce crop surpluses and cut government spending on erosion control and farm production subsidies.

USDA soon will announce details on the amounts of land that farmers have offered to put into the reserve this year, along with their proposed prices for 10-year contracts to idle their most erodible fields. Depending on costs to the government, as much as 5 million acres could go into the reserve in 1986.

A coalition including the Sierra Club, the American Farmland Trust, the National Audubon Society and the National Wildlife Federation wrote yesterday to Appropriations Committee members, urging defeat of Whitten's changes.

"These proposals would undermine the soil conservation reserve and penalize small family farmers, while favoring the bigger operators," said Daniel Weiss of the Sierra Club.

Whitten's changes include limiting a farmer to enrolling no more than half of his cropland, no matter how erodible, in the reserve, and limiting reserve payments to one-third the value of the previous year's crop -- a provision that Weiss said would penalize farmers who suffered drought, flood or losses due to pests.

No such limits are now in the program.

Whitten could not be reached for comment, but in the past he has expressed fears that lower crop production that could result from large-scale land retirement might impair U.S. efforts to regain lost export markets. He also has been critical of Reagan administration plans to finance the reserve by ending some conservation programs that he has strongly supported.

Earlier this month, Whitten threatened to limit funding of the conservation reserve as a protest against the administration's plans to cut other programs. He dropped the threat when Senate negotiators in a conference committee refused to go along with his plan.

"Our budget leans very heavily on the new conservation reserve authorities, but it would eliminate some of the programs that Whitten rightly considers himself the father of," the USDA official said. "He views some of these new authorities as a threat to those old programs."