What a difference two years makes.

In 1981, President Reagan relegated the Export-Import Bank to his budgetary ash heap, describing it as "another major business subsidy." To show he was even-handed in demanding sacrifices from the rich as well as the poor, he called for a one-third cut in its lending authority "because the prime beneficiaries of taxpayers' funds in this case are the exporting companies themselves--most of them profitable corporations."

Now the White House has changed its tune and resurrected the Ex-Im Bank. In his State of the Union message Tuesday night, the president said greater loan guarantees and, possibly, increases in Ex-Im Bank direct lending authority are needed to boost the sale of American products overseas--exactly the argument used by critics of Reagan's 1981 policy that crimped Ex-Im funding.

Those critics ranged from Stuart E. Eizenstat, a Washington lawyer who was chief domestic policy adviser to President Carter, to Malcolm T. Stamper, president of the Boeing Co., America's No. 1 export money-maker. They argued that a strong Ex-Im Bank is needed to help provide cut-rate financing for purchases of American goods that is competitive with financing offered by other countries.

Eizenstat, who represents a coalition of 41 businesses, 14 labor unions and 14 governors opposing the Reagan administration policy on the Ex-Im Bank, said yesterday the president changed his mind because he realized that exports can help the economic recovery. "They are demonstrably a job creator, a good antirecession tool," Eizenstat said.

"It's really been an education process," Stamper said. "It's been my perception that they White House officials really didn't see the impact it was having."

Moreover, the administration's attitude toward the Ex-Im Bank had become a political issue in some congressional races last fall, and Walter F. Mondale, the leading contender for the Democratic nomination, was blaming Reagan-ordered cuts in bank funding for the loss of foreign sales.

Boeing, for instance, argued that cut-rate financing for the Airbus, made by a consortium of European governments, was costing it sales for its new 767 wide-body passenger jet. Stamper said his company could have sold $1 billion more in planes last year if there had been a more aggressive government approach to export financing.

Westinghouse was reported by trade officials to have lost nearly a $100 million order to sell turbine generators to South Korea because it could not match government-subsidized financing offered by a French company.

The financing difference on a hundred-million-dollar deal if one country offers a subsidized loan 3 to 4 percent below market rates can amount to tens of millions of dollars, Eizenstat said.

"We could point to major export losses because of a lack of competitive financing," one administration trade official said. Another described financing practices of other nations, especially the Europeans, as predatory.

U.S. Trade Representative William E. Brock and Commerce Secretary Malcolm Baldrige were reported to have joined Bank President William Draper III in taking the lead, against opposition from David A. Stockman's Office of Management and Budget and the Treasury Department, to give greater support to the Ex-Im Bank. One knowledgable source said the final decision was made by the president.