The administration, conceding that President Reagan's election-eve farm-credit aid program has not worked, announced a revision yesterday aimed at helping thousands of faltering farmers obtain loans for spring planting.

Agriculture Secretary John R. Block said the new debt-restructuring package will "help farmers through their temporary difficulties . . . and make it possible for many operators to stay in business."

But the new provisions -- the most important of which would allow qualifying farmers to stretch out old loan repayments -- ran into instant skepticism from rural bankers and Republican farm-state legislators, who said the administration was acting too late and offering too little to prevent economic chaos in rural America.

"There's nothing new. These are totally insignificant changes," an official of the Independent Bankers Association of America, which represents most of the rural banks, said yesterday.

Sen. Nancy Landon Kassebaum (R-Kan.) said, "I cannot embrace a program that offers too little assistance and too little hope, a plan that is inadequate to meet the needs of the farm economy."

While Block said the new program, which also would allow banks to be more lenient toward farmers, will enable many farmers to avoid foreclosure, he again linked long-term agricultural prosperity to adoption of the administration's forthcoming proposals to rework the basic farm price support system, now costing billions of dollars a year.

But advance congressional reaction to the proposals has been caustic and, in response, the administration blinked yesterday on one part of its proposal -- a plan to kill tobacco price supports. Block told tobacco-state congressmen who were threatening to sabotage the farm bill that he will send the tobacco proposals to Congress separately.

The political furor over the farm debt and tobacco was inflamed further when budget director David A. Stockman this week denounced farm subsidies and blamed farmers and lenders for creating the credit crisis.

White House spokesman Larry Speakes, attempting to take the edge off the remarks by the head of the Office of Management and Budget, said that Reagan "is sympathetic to the difficulties farmers face in obtaining credit for the planting season" and that the debt-restructuring program announced yesterday demonstrated that.

That aside, Stockman's remarks left some legislators in shock. Sen. Mark Andrews (R-N.D.) charged that the budget boss was "practicing a Marie Antoinette economics that is totally nonproductive for our nation's economy."

Andrews and other Republican farm-state senators, many of whom face sensitive reelection campaigns next year, were instrumental in pressuring the administration to recast the debt-assistance plan unveiled by Reagan last fall on the eve of a campaign foray into farm country.

Block conceded yesterday that the plan's federal guarantee of private loans to farmers has not worked. Of $650 million made available for the guarantees, only $25 million has been used.

The other part of the program allowed the Farmers Home Administration (FmHA), which provides about 11 percent of farm credit, to set aside up to 25 percent of a farmer's debt for five years without interest. Block said that is working, but Agriculture Department figures indicate that only about 3,900 borrowers (out of more than 200,000 nationally) have been granted setasides.

Key changes announced yesterday included:

* The government will allow a bank to reduce interest as well as principal on a farmer's guaranteed loan, which Block said will attract more banks, particularly those facing financial stress.

* The USDA will guarantee up to 90 percent of operating loans to farmers previously served by lending institutions that went broke. Block said this would give many farmers a new "credit home" and allow them to carry on.

* The Treasury, the Federal Reserve System, the comptroller of the currency and the Federal Deposit Insurance Corp. will adopt a policy of "forbearance" to avoid pressuring rural banks into shutting off financially troubled farmers and small rural businesses.

* The USDA will send teams of FmHA lending officers to provide credit assistance to farmers in areas where commercial banks or institutions of the Farm Credit System have been liquidated. The department also will help operate "credit hot lines" to provide information, and will liberalize its hiring standards to recruit more lending specialists.

The Independent Bankers Association of America remained cool toward the administration's changes. The bankers had sought up to $3 billion for the loan guarantees and federal sharing of forgiveness of as much as 4 percent of a farmer's loan.

"The $650 million is the same amount that was in place last September and that has been sidetracked for 4 1/2 months," the association's Weldon Barton said. "This relatively small amount continues to frustrate the program. It is penny wise and pound foolish for the administration not to seek the extra money."