AMERICANS ARE now borrowing more heavily than at any time in their history. This great surge of borrowing began four years ago and continues to run strongly. It is being led by the federal government, but there is much more to it than the budget deficit. Corporations are borrowing heavily, much of the borrowing being related to take-over strategies. Private individuals are borrowing heavily, pushing the country's standard of living well beyond any level that it is currently earning -- and the difference is visible in those growing foreign trade deficits.

How dangerous is this accumulation of debt? The answer starts with a little history. From the end of World War II to the early 1980s, there was remarkable stability in Americans' total debt, public and private. It stayed at an almost constant ratio to the size of the national economy, just under one and a half times the gross national product. The United States came out of the war with a high federal debt but low levels of business and personal debt. Over the next three and a half decades, federal debt dropped in relation to GNP while, at precisely the same rate, private debt rose to finance industrial growth, housing and, increasingly, private consumption.

That changed after 1981. Both public borrowing and private borrowing turned in the same direction -- up. The total accumulation of debt is now significantly larger, in relation to GNP, than at any time in the past generation.

If this borrowing were being financed entirely by Americans, the consequences would be simpler and less disquieting. Economists used to be able to dismiss doubts about rising debt by observing that it was merely a matter of Americans lending money to other Americans within the closed system of the national economy. That's no longer true. A substantial amount of this borrowed money now comes from abroad.

As Americans become increasingly dependent on foreign financing, the foreign investors will become more important to the American economy. Collectively they will have great influence over two of the fundamental determinants of American prosperity, the interest rates and the dollar's foreign exchange rates.

Rising debt itself is not necessarily a threat. It's the way in which it is being financed that deserves attention and concern. The original Reagan economic program of 1981 was supposed to encourage greater savings and investment. Instead it has generated high consumption and a dismayingly low savings rate -- with the gap being filled, at the moment, by Japanese, European and Latin American lenders. Of all the miscalculations in the Reagan program, in the years ahead this one is likely to prove the most troubling.