BANKING is getting riskier. That warning was repeated the other day by L. William Seidman, who presides over the deposit insurance system. He offered the somber prediction that the banks' deposit insurance fund would run a deficit of about $2 billion this year, meaning that it will pay out that much more in back failures than it will collect in premiums. The fund is still solvent -- unlike its counterpart that insures S&L deposits -- but this will be the third consecutive year in which it has suffered substantial losses. Last year there were more bank failures than in any since 1933, and this year apparently won't be much better.

It's been evident for a decade that the structure of banking, and financial services generally, needs fundamental reform. The present laws are dangerously obsolete. The S&L disaster is now forcing decisions. Congress has ordered the Treasury to carry out a study of the deposit insurance system, and it's to be published at the end of the year. It will apparently be an outline for the sweeping financial reforms that are becoming urgent.

The Senate Banking Committee has been holding hearings in an effort to get some idea of the direction in which things are moving, and last week the secretary of the Treasury, Nicholas F. Brady, set out in very general terms the principles that he's following. Now Mr. Seidman has followed him, speaking in much more explicit detail.

There are enormous political collisions ahead over the central questions of the restrictions on the banks, and the rules for competition between them and the other financial enterprises -- particularly the investment banking firms -- that have been cutting into their traditional business. Banks need greater freedom of action, Mr. Seidman said, but it's got to be reconciled with the taxpayers' need for for greater protection from runaway liabilities like those in the S&L cases.

Beyond the technicalities of banking law lies a large and unpleasant reality that this country is having trouble acknowledging. Since the last recession nearly eight years ago, Americans have been living -- most of them prosperously -- on an enormous surge of public and private borrowing. The great borrowing boom has created a mountain of debt, and not all of that debt is going to be repaid even if the economy keeps growing steadily. If there's a recession, there will be a real danger of cascading defaults. The Treasury and Congress need not only to construct a financial system in which competition is fairer and more efficient than today. They have to construct a system that's also safer -- that recognizes the excesses of the 1980s and prevents a repetition in the 1990s.