The second presidential debate came and went without a serious word from either candidate about the international issue that has the greatest direct impact on the greatest number of Americans.

That issue is the threat to the economic health of the United States from the unstable spending, taxing and borrowing policies of the federal government and the effect of these policies on the dollar and the worldwide commitment to open trade.

Assuming that Amageddon remains an issue of religious interest and American marines aren't put ashore in Lebanon or Central America, the gravest foreign policy challenge for 1985 likely will be the overvaluation of the dollar.

Although the opinions of economists are not unanimous on the subject, the vast majority of experts agree that the high value of the dollar compared with other currencies is doing serious -- and in many cases lasting -- damage to American producers of goods that must compete against foreign imports and to U.S. companies that try to sell goods and services abroad.

A tide of imports is rolling into this country, rising at an unprecedented annual rate of nearly 80 percent in the past few months. The continued strength of the dollar, which inflates the costs of American goods and services relative to foreign competition, may be the single most important reason for the import surge. But on many other fronts, the basic nature of trade competition is changing.

The commitment of industrial nations to free trade, imposed by the United States after World War II and vitalized by American willingness to import from all sides, was designed for traditional trade in steel, oil, textiles and other minerals and commodities based on competitive advantages in resources, geographic location and labor costs that shifted slowly from one nation to another.

If the industrial world has entered an extended period of slower economic growth -- as is the assumption in many large U.S. corporations -- then the competition in basic commodities will become tougher than ever. And the commitment to free trade is now being tested by forces unimagined a generation ago -- the explosive growth and migration of technology and capital is rapidly turning a number of developing nations into high-tech manufacturers.

But President Reagan and Walter F. Mondale weren't asked -- and didn't volunteer -- any useful insights in their debate about how they would deal with these challenges.

A somewhat clearer picture is presented in a booklet on the candidates' economic positions, solicited and published by the LTV Corp. in a booklet, "The Candidates, Where They Stand."

In the response supplied LTV by the White House, President Reagan begins by noting how central trade has become to the health of the U.S. economy: American exports of goods and services now account for 10 percent of the gross national product, twice the share in 1960.

"We believe, quite simply, that given an open and fair world trade system, American workers and businesses have the talent, the initiative and the innovative ability to succeed in the international marketplace," Reagan said.

He noted, correctly, that the reduction in inflation and the improvement in productivity since 1981 have removed two of the worst handicaps faced by American producers when he came to office. And he rightly took credit for the business tax changes that pumped cash into corporations and finally revived capital spending.

Not surprisingly, he failed to mention that the price for these improvements included the worst recession since the Depression of the 1930s. And he overlooked the contribution his 1981 tax cut made to the huge federal budget deficits that now stretch over the horizon, which, in turn, are a major factor in the high value of the dollar.

"If U.S. producers are harmed by unfair competition, we will take remedial vigorous action. But we will continue to seek to break down trade barriers, not build them up . . . ," Reagan said, accusing Mondale of trying to do the opposite.

In his response, Mondale noted that 1983 was "the worst trade year in American history," producing a commercial trade deficit of nearly $70 billion, and noted that this year's deficit may reach $125 billion.

"American businesses, workers, and farmers can compete against any businesses, workers and farmers on earth," Mondale said. "Today, however, they're often asked to compete against foreign government." U.S. producers are being frozen out of market after market by foreign quotas, government subsidies, and trade restrictions, he said.

Mondale's remedy is to raise the stakes by countering those trade restrictions by tougher U.S. restrictions -- until this country's trading partners move toward freer trade. "I believe in free trade, but I also believe that it's time for a much stronger trade policy that insists on fairness.

"We must counteract foreign trade barriers. We must let other countries know that, if necessary, we will combat their subsidies until they agree to a much more fair and open system of international trade," he said.

Unlike Reagan, Mondale has committed himself to a tax increase, if elected, to begin a reduction in federal budget deficits that would permit interest rates to decline and ease the dollar downward.

But Mondale has lost the opportunity in this campaign to tell the truth to his audience -- that even with a lower dollar, the United States has lost the ability to compete in price and quality in many areas where it once dominated and must focus all the harder on the fights it can win without the need for protection.