The Bush administration is proposing that Congress authorize it to buy billions of dollars in federal student loans to make sure the nation’s credit crunch does not block borrowing for higher education.

The proposal, outlined in a letter to be sent Wednesday to members of Congress from the Education and Treasury Departments and the Office of Management and Budget, endorses a provision in a bill passed by the House this month. It is the latest sign of deepening concern that the combination of tougher standards for borrowers, lender withdrawals from federal student loan programs and falling housing prices poses a threat to families struggling to cope with rising tuition. By buying loans, the government would provide capital to lenders to make new loans.

“I want to make double-dog sure that we have the tools necessary” to make sure federal loans continue to be available, Education Secretary Margaret Spellings said in an interview on Tuesday. “If we don’t need to use them, so be it.”

Ms. Spellings said she was not sure under what circumstances the Education Department would actually begin to buy the loans. “This is dynamic,” she said. “We don’t know.”