IT MUST BE THE combination of heat and the recession, because we seem to have entered the silly season on the protectionist front. Here in Washington, the Giant Food chain has offered employes who buy a U.S.-made car a tax-paid cash gift of $100.

And up in New York, labor's Amalgamated Bank took space on the Times' op-ed page to advertise a special interest rate on loans to "people who buy American-made cars -- cars built by American workers."

Reminded on the telephone of the old adage that trade is a two-way street, Jacob Sheinkman, president of the Amalgamated Clothing and Textile Workers (the union that owns the bank), snapped:

"I'm well aware of export problems. But the American auto industry is being battered by imports. It needs a breather, and we want to give the (domestic companies) a shot in the arm."

Jesse Jackson, a leading black spokesman, labels such protectionist devices aimed at Japan as a subtle form of racism.

But don't jump to cheer. Jackson has what he thinks is a better idea.

In a letter to the Jaspanese ambassador in Washington, Jackson warned of a boycott of Japanese products unless Japanese companies in the United States hire more blacks, deposit money in black-owned banks, create black dealerships and buy a "significant" amount of advertising in black journals.

Jackson, head of Operation PUSH, thus affirms that he is no racist: He is merely interested in the color of their money.

The only encouraging news on the trade front -- in a week in which the Carter administration continued to pursue a protectionist policy in the guise of "reindustrialization" -- was the gutsy refusal of the International Trade Commission to decide a Japanese auto quota case before the election, as President Carter had urged it to do.

Unwisely, Carter had suggested to the ITC that it accelerate its decision on an import injury complaint brought by the United Auto Workers. But the ITC, in what amounted to a restatement of independence, agreed to advance its decision only from Dec. 12 to Nov. 24, three weeks after the election. What the president then does can be detached, at least theoreticaslly, from political considerations.

With the nation moving into what may well be its worst economic downturn since the Big Depression, wiht 325,000 UAW members out of work, the problem of auto imports was bound to become emotional.

But the auto industry is not in trouble because Japanese cars are in great demand. The auto industry is in trouble here because it missed the boat in assessing the long-term nature of the oil price/-supply problem. Now, it will take 18 months to two years for the retooling necessary to make Detroit fully competitive with Tokyo.

Sheinkman and Amalgamated Bank President Edward M. Katz do not hold the industry blameless. But they say that the workers who can least afford it suffer for management mistakes. "Meanwhile, we have to decide whether we want to maintain a manufactuing base in this country," Sheinkman said. "The problem is the same for steel and textiles."

What Sheinkman and Katz forget in their well-intentioned, if misdirected, approach is that union members are also working at jobs dependent on U.S. exports. According to Assistant Treasury Secretary C. Fred Bergsten, one out of every eight manufacturing jobs in the United States produces for exports. And one out of every three acres of U.S. farmland produces for export.

It's easy in a period of strain revolving about inflation, high unemployment and slow growth to forget these basic, overriding truths and to lapse into a protectioist stance. This means reinforcing inflationary trends by limiting the availability of more efficiently produced goods.

Trade restrictions (or incentives) are no substitute for the right national economic policy: to encourage a shift away from lowwage semi-skilled industries, where the United States is losing out to dozens of developing nations. What is needed is an emphasis on the high-technology stuff where the United States is now among the leaders.

Admittedly, it's natural for union leaders to seek to build a fence around their own preserve. The same problem exists in every industrial country in the world as the price of oil skyrockets, increasing the cost of imports and pressuring everyone into beggarthy-neighbor policies.

This is the overriding economic issue of our day. No leader of any major nation is facing up to it.