The administration has made a $100 million decision to delay until after the November elections a new fee that will be charged dairy farmers in an effort to cut the soaring cost of the federal dairy price-support program.

As recently as Tuesday, Agriculture Department officials had been telling Congress the new fee would take effect Oct. 1. But yesterday Secretary John R. Block announced that farmers will not be assessed until Dec. 1.

The two-month delay means that USDA will forgo an estimated $100 million that would have been used to help defray costs of the dairy program, which is expected to cost about $2 billion this year.

One of the first reactions to the USDA decision came from Rep. Tom Harkin (D-Iowa), chairman of the House Agriculture subcommittee on livestock, dairy and poultry, who accused the administration of political double-dealing.

"It shows the hypocrisy of the Reagan administration when it says it is cutting government spending," Harkin said.

"The only reason they have done this is to save the seats of a few Republican congressmen from dairy districts -- and at a cost of $100 million to the taxpayers."

A group of Republican congressmen, led by Rep. Steven Gunderson of Wisconsin, who has more dairy cows than constituents in his district, objected to Block after USDA officials briefed legislators on Tuesday about the plans to begin collecting the fee Oct. 1.

At USDA there was some division of opinion about the political side of the decision, but Deputy Secretary Richard E. Lyng, taking credit for the postponement, said, "We have not been guided by political considerations." Lyng said Oct. 1 had been set as the starting date, but after word of that got out, he and other officials had second thoughts.

"A lot of objections started coming in from places we hadn't thought about and at least three groups had hired lawyers to challenge us."

He said that the department, faced with possible court action against its plans to begin collections on an emergency basis, decided to provide a 45-day comment period after its proposed regulations are published in the Federal Register.

Block's executive assistant, Raymond D. Lett, said that top-level discussions touched on "a little bit of everything," including the political aspects of delaying collections until after the November election.

But, Lett added, "We just felt as though those folks who were complaining were entitled to the normal comment period."

At the Office of Management and Budget, which has spearheaded the administration's push since early last year to curb rising costs of the dairy program, spokesman Edwin L. Dale Jr. acknowledged that the administration will be giving up some potential budget savings.

And he confirmed that "there was some political pressure," but he said OMB had no role in the final decision.

The dairy assessment was approved by Congress last month as part of its budget reconciliation package. The fee of 50 cents on every 100 pounds of milk will have the effect of dropping the current price support of $13.10 per hundredweight to $12.60.

The idea behind the assessment is to cut the farmers' guaranteed price and encourage them to reduce their milk production, which has been increasing for three years and glutted federal storage facilities with dry milk, cheese and butter. CAPTION: Picture, JOHN R. BLOCI. . . action to cost $100 million.