THROUGH MOST of this year, the stock market and the foreign exchange market have been marching to very different music. The stock market has been full of pep and optimism, stepping smartly along to drums and bugles. The foreign exchange market, in great contrast, has limped along in constant fear of pitfalls and ambushes. Now, for the past couple of days, the two markets have been converging in spirit and outlook -- at the expense, unfortunately, of stock prices.

It started with the publication of this country's August trade deficit, a much higher figure than most people (including ourselves) had expected. Foreign exchange traders saw that there was no real improvement in the imbalance that is flooding their market with the dollars being earned by other countries' exports here. If foreigners do not buy those dollars for investment purposes, the law of supply and demand will take over, and the dollar's exchange rate will once again drop. Since people in the markets know the U.S. government has promised to prevent that drop, they assume it will have to do what it can to make the dollar more attractive to those foreign investors. That means raising interest rates. Higher interest rates mean falling profits for business, and that's the prospect to which the stock market has been reacting.

Why didn't it happen a month ago, when the even higher trade deficit for July was announced? The monthly statistics are notoriously unreliable, and perhaps people were waiting for further confirmation. On Tuesday they got it.

These numbers also give ominous weight to the warnings of some experienced watchers -- Paul Volcker, for one -- that devaluing the dollar will not alone cure the American trade deficit. If the dollar is not to fall much, much farther, this country will now have to use other methods to get that deficit under control.

American consumption is going to have to come down. The Reagan administration's great consumption boom, fed by its budget deficit, is going to have to come to an end. If the country does it voluntarily and purposefully, the pain will be slight and widely distributed. If it's involuntary, the result will be what's known as a recession.

For all the tremendous losses that the falling market represents, it may serve one deeply useful purpose. It may succeed in convincing a lot of influential Americans that the domestic economy can't be insulated from the international markets -- that the American deficits are a real and imminent threat to American prosperity, and that remedies cannot safely be postponed until another president has taken office