BY NOW MOST Americans must be aware that New Year's Day will bring a stiff little tax increase to everyone who earns wages - in fact, we have a number of letters on the subject today. It's the payroll tax that pays for Social Security, and this increase is already in the law. The bill now grinding along through Congress lifts substantially the schedule of further increases over the next 10 years. The new schedule will hit particularly hard those people whose earnings lie in the upper-middle range, from about $19,000 a year up. These changes are going to exacerbate all the ambiguities in Social Security policy, and do some damage to the politcal consensus that has, until now, surrounded the system. Another effect will be, unfortunately, to make labor increasingly expensive to the employer at a time when unemployment is already very high.

In legal terms, the payroll tax is divided between the employee and the employer. That division is, of course, a pleasant legal fiction. In economic reality, the employee actually carries both halves. Employers treat both shares as part of the employee's wages, along with other payroll taxes and fringe benefits. It's important to keep that truth in mind, in order to see what the consequences of last week's Senate votes will be.

The Senate has now voted to increase the so-called employer's share much faster than the employee's. In the past they have been equal. It's not just a matter of concealing from the uneasy taxpayer the true dimensions of the tax on his wages. This change makes a significant difference in the underlying principle of Social Security. The law links a citizen's retirement benefits to the payroll taxes that he or she has paid - but only to the payroll taxes paid as the legally defined "employee's share." Raising taxes means raising benefits, in years ahead. But raising the so-called "employer's share" on above-average wages, as the Senate bill would do, is a device for raising those taxpayers contributions without increasing their benefits.

With that change, the Social Security system will look a little like insurance, and a little more like welfare. It has always contained a mixture of those clements, of course. Present benefits are not directly proportional to tax contributions. The system already tilts a little to the advantage of people with low wages. But if the Senate should prevail, that tilt will become much more pronounced. Above-average wage earners will be paying substantially more in taxes than they get in benefits. That, no doubt, is the way that it ought to be. But the argument for financing Social Security with the regressive payroll tax has always been that it is basically an insurance plan. As Congress makes it less an insurance plan, the case for shifting part of the load to the income tax becomes stronger.

The earnings limit demonstrates a similiar confusion. If you retire on Social Security, but take part-time work that brings you more than $3,000 a year, your benefits will be cut on grounds that you don't need them. But if you are fortunate enough to retire with $100,000 a year from private pensions and securities, you can draw your full Social Security on grounds that you aren't working. The logic, you will perceive, is a bit clouded. Is Social Security an earned annuity, or is it to be distributed on the basis of need? Sometimes it's one, sometimes the other. As the system rapidly gets more expensive, and more important to American life, these anomalies become less tolerable.