The Reagan administration has moved to knock out a key component of Congress' plan to reduce the federal budget in fiscal 1988, saying it would cost the goverment more in the long run than it would save next year.

The Treasury Department has determined that it cannot allow early repayment of $7.2 billion in Rural Electrification Administration loans, which would have counted as federal revenues. The prepayments would have an adverse impact on the Federal Financing Bank, which raises funds for a variety of agencies, the Treasury decided.

Capitol Hill aides who were briefed on the issue Tuesday said yesterday that the Treasury has used its statutory discretion to disallow refinancing the loans, most of which carry high interest rates and probably would be refinanced in the private sector at lower rates. The government would lose a net $2 billion in interest revenues, the Treasury has determined. Many of the loans carry 12 percent and 13 percent interest rates.

The short-term revenues generated by the early repayment of the electrification loans, which are used by rural cooperatives for construction of generation and transmission facilities, represent about one-fifth of the $36 billion in deficit reduction called for in Congress' $1 trillion budget.

By statute, the REA may permit the prepayment of slightly more than $2 billion in rural electric loans unless the treasury secretary finds it would harm the financing bank. A fiscal 1987 appropriations bill signed by President Reagan Saturday sharply increased the amount that can be prepayed.

But it did not remove the power of the secretary of the Treasury to disallow loan prepayments.

Sen. Patrick J. Leahy (D-Vt.), chairman of the Senate Agriculture Committee, yesterday sharply criticized the decision. "While the president is out making speeches on economic responsibility, his bureaucrats are irresponsibly undermining the budget agreement," Leahy said.

Senior officials at the Treasury Department could not be reached for comment on the decision. But an Agriculture Department official said it was his "understanding that the secretary of the Treasury will be consistent" with two previous decisions disallowing refinancings determined to have a detrimental impact on the financing bank.

However, Capitol Hill aides said the Treasury decision would likely have only a temporary impact on the Democratic-controlled Congress' ability to go forward with its deficit-reduction plans. The aides said that Congress would move to correct the problem when it adopts separate legislation necessary to implement the $1 trillion budget.

The REA currently holds $18.8 billion of loans, which have an average interest rate of about 10.9 percent. The budget assumed that it would be advantageous for any borrower with federal rates above 9 percent to refinance. However, rising private rates have changed that assumption, lowering the expected yield to about $6 billion.