THE GREAT BORROWING boom of the past several years has pushed Americans' total debt, public and private, to the highest levels in the country's history. This surge of debt has started a widening debate over financial safety. On one side are some of the federal banking regulators, most notably the Federal Reserve Board. On the other are the bankers, legislators and government officials who believe in the principle of deregulated financial markets.

Earlier this month, over a great deal of articulate opposition, by a three-to-two vote, the Federal Reserve Board restricted the use of junk bonds -- bonds carrying high interest, and high risk -- in corporate takeovers. Some of that opposition seemed to have been organized from the White House by Donald T. Regan, the president's chief of staff. It was an odd occasion for a controversy as fierce as that one, for the new restriction itself is extremely narrow. But it touched that central quarrel between the regulators and the deregulators.

Now the Federal Reserve has proposed another change in the rules, this one with far wider application. At present a bank's capital is required to be a flat 6 percent of its assets -- its assets being, chiefly, its outstanding loans. The Federal Reserve asks whether the capital requirement should not vary with the risk of the loan, with a high requirement for a high-risk loan and a low requirement for a safe one.

The bankers in general don't like the idea. They fear that it would put them at a disadvantage with the savings and loan associations which, notoriously, have no such requirement. The banks also argue that, by categorizing loans, the new rule threatens to impose a kind of credit allocation that could make it hard for borrowers in certain troubled businesses -- farming, to take a current example -- to find money.

The answer is that a bank examiner can be asked to distinguish between a farmer who is assuredly able to repay a loan, and a farmer with heavy debts whose prospects are less certain. The banking system is now under strain. More banks failed last year than in any year since 1933. As the price of oil falls, that strain will increase.

The comparison with the savings and loan associations illustrates the point. In the loosely regulated S&L industry, associations that suffered losses have often tried to recoup through high-risk investments. Unfortunately the result has usually been further losses, larger than ever. There are now hundreds of bankrupt S&Ls still in business, their losses far beyond the capacity of the federal S&L deposit insurance fund to cover them.

The Federal Reserve is trying to prevent a similar pattern of desperate gambles among banks. The danger is real, and in this case careful regulation is going to be essential.