As recently as 1970, few Americans cared about trade. With exports amounting to a mere 6.5 percent of the gross national product, most companies focused on what they thought was an ever-expanding domestic market; overseas sales were not considered a key factor in the U.S. economy.

All that has changed as America's stores have filled with foreign products and the U.S. merchandise trade deficit hit record heights last year and is still climbing. Government economists expect it to reach $70 billion this year and $100 billion in 1984, with no turnaround in sight. And Alfred E. Eckes, chairman of the International Trade Commission, cited published figures warning of a $174 billion deficit by 1990.

Commerce Department figures show American exports bottomed out in the fourth quarter of 1982, and picked up in the first half of this year. Even so, they ran $14.4 billion less, on an annual basis, than last year's depressed level, while imports increased at an annual rate by $10 billion. There has, furthermore, been no growth since 1979 in U.S. exports to recession-hit Western Europe.

"There's no light at the end of the tunnel. There's no reason to think the trade deficit will improve with the dollar so overvalued," said economist C. Fred Bergsten, a former Treasury official who now heads the Institute for International Economics.

It is well recognized now that trade means jobs, and that with 9.5 percent unemployment, the United States is losing out. Commerce Department economists estimate that every billion dollars in exports equals 25,000 jobs, meaning that last year's $43 billion merchandise trade deficit cost the country at least one million jobs.

"When the volume of manufacturing exports was growing between 1977 and 1980, that growth accounted for 30 percent of the increase in U.S. private-sector employment," concluded Commerce Department economist Lester A. Davis in a May study of jobs and trade.

"When total export volume decreased during 1980-'82," he added, "that decrease accounted for 40 percent of the rise in U.S. unemployment."

And looking ahead, Bergsten estimated that the $100 billion trade deficit predicted for 1984 could mean two million lost jobs. "The trade deficit keeps the unemployment rate a couple of points higher."

It also feeds the forces of protectionism in Congress, which is considering a number of bills designed to limit imports. The most prominent of them is a domestic content bill that is aimed at cutting U.S. sales of Japanese-made cars.

That bill has the strong support of organized labor, although the Reagan administration has labeled it the most protectionist measure before Congress since the Smoot-Hawley Act of the 1930s, widely blamed as a precipitating cause of the Great Depression.

As a result of America's trade deficit and the resultant loss of jobs, trade is becoming a major presidential campaign issue for the first time since the 1884 election, when Grover Cleveland, a Democratic free-trader, narrowly defeated Republican John G. Blaine, who ran on a high-tariff platform. This year also marks a historic reversal of traditional American political attitudes, as the Democrats have taken the usual GOP role of protectionism while the Republicans have assumed the free-trade mantle.

Despite organized labor's preoccupation with domestic content legislation, imported cars aren't the only foreign-made goods Americans are buying.

No TV sets are made in America any more even though they bear labels of U.S. companies; Bloomingdale's sells "Made in Korea" summer suits, and a leading U.S. tuxedo maker has complained on Capitol Hill that imports from Bulgaria are cutting into its sales.

Since 1960, Commerce Department figures show, imports have more than doubled as a percentage of domestic consumption--from 5.8 percent to 12.5 percent in the first half of this year.

ITC Chairman Eckes, an economic historian, sees "disturbing" trends in "the shifting composition of our trade.

"We are importing more and more primary products and importing more and more manufactured goods," he said in a speech last month to the National Press Club.

"This, incidently, is the traditional definition of a less developed country."

Eckes cited American trade with Japan, where in 1982 the United States' five leading exports were corn, soybeans, wheat, cotton and coal and its five leading imports were autos, trucks, video recorders, oil well casing and motorcycles. Further, he said, the same pattern is being repeated with the newly industrialized East Asian nations of South Korea and Taiwan.

"Some might describe the emerging relationship as reminiscent of the colonial trade pattern this country had with Great Britain in the 18th century," when, for most of the time, America was a British colony, said Eckes.

While Bergsten assigns major blame for the trade deficit to the overvalued dollar, David Lund, senior international economist for the Commerce Department, adds other factors: The global recession that caused even the industrialized nations of Europe to curtail their imports; government deficits here and the swollen $600 billion debt among Third World nations that traditionally buy 40 percent of the United States' overseas sales.

"The United States lost $9.4 billion in sales to Mexico alone from 1981 to the first half of this year," said Lund, because of drastic reductions in overseas purchases forced by that country's massive, $90-billion foreign debt.

The drop in exports to Mexico alone cost 225,000 American jobs, excluding the loss of employment in American border towns that depended on Mexicans coming over to shop. Because of the fall in retail sales, unemployment has reached record levels in many Texas cities; Laredo, for instance, reached the highest unemployment rate in the nation, more than 27 percent.

There is a widespread belief among economists that Mexico and Brazil--which also has international debts in the $90-billion range--will have to begin importing soon if for no other reason, said Lund, than to acquire the capital equipment and supplies they need to produce manufactured goods they can export.

As of now, however, they are carrying trade surpluses as their sales abroad--needed in order to pay off the ballooning interest on their huge international debts--exceed the constricted imports.

The surge of imports into this country this year was expected by economic analysts as the United States led the rest of the world out of the recession and Americans found themselves with more cash to buy goods. The swollen dollar, moreover, makes imported goods less expensive here, while American products are more costly for overseas purchasers.

A soon-to-be published study by John Williamson, a senior fellow at the Institute for International Economics, found the dollar was overvalued by 24 percent when measured against other nations' currencies and compared with the relative purchasing power of the different countries' money.

"That's like putting a tax of 24 percent on all exports and giving a subsidy of 24 percent for all imports," said Bergsten.

The English pound was also found to be overvalued, but the Japanese yen was 6 percent under what its value should be, as was the French franc, according to the Williamson study. The German deutschemark was found to be 5 percent under its value.

Japan, which had a $16.8 billion trade surplus with the United States in 1982 that is expected to grow to $22 billion this year and $27 billion in 1984, benefited from having the dollar 20 to 25 percent overvalued against the yen, Bergsten said. Williamson's study shows the yen should be valued at 205 to the dollar instead of the present exchange rate of about 250.

Bergsten blamed the Reagan administration for "having done absolutely nothing to avoid the problem" and said high interest rates and its refusal to intervene in currency markets "will be a major source of disruption over the next couple of years."

He said, moreover, that there are few good alternatives--other than a tax increase--for lowering interest rates, which would stem the flow of capital to the United States, thus lowering the value of the dollar. President Reagan, however, steadfastly has refused to consider any tax increase.

Japan could help in the case of the undervalued yen, he said, by issuing "administrative guidances" to limit the outflow of capital. Then joint U.S.-Japanese intervention in currency markets could have some effect but, Bergsten ackowledged, that would be "a second-best policy" to American tax increases.

Nonetheless, he said the the United States and Tokyo should be discussing such moves in advance of Reagan's November visit with Prime Minister Yasuhiro Nakasone.