EACH ADMINISTRATION develops its own vocabulary of dismal euphemisms, and Mr. Reagan's White House now speaks of "revenue enhancement." That, as you perceived some time ago, is a tax increase. The president favors only a modest degree of revenue enhancement. The Senate Republicans think that something more substantial is going to be necessary, and they are right.

Sen. Robert Dole, the chairman of the Finance Committee, suggests a broad tax on energy. That at least has the virtue of pushing the country toward conservation. The majority leader, Sen. Howard Baker, briefly floated, in a gingerly way, the possibility of a national sales tax. That one is destitute of any virtues at all except--since there's nothing whatever to be said for it--that it lends itself to brief and forthright commentary.

You will observe that the personal income tax, which was massively cut last summer--dis-enhanced, as the White House would put it--is progressive. It takes a higher proportion of larger incomes than of small ones. The substitutes now under discussion in the Senate are regressive, taking larger proportions of small incomes than of large ones. Other examples, on a smaller scale, are the increases, also under examination in the Finance Committee, of the excise taxes on beer, cigarettes, and so forth.

A central principle of federal taxation, as old as the income tax itself, is being reversed. If you accept the idea that a tax system reflects a society's working definition of equity, you will acknowledge that these changes reach far beyond mere technical fiscal tinkering.

Last summer's tax cut was shaped by the opinion that the mediocre performance of the national economy in the 1970s had been aggravated by excessive taxation of those people with wealth who save and invest. This theory holds that a little more inequality in incomes will make the whole economy grow faster, benefiting everyone, including those at the bottom. That theory, it might be observed, is based on faith alone, since there is no evidence to fortify it. But while faith in Mr. Reagan's economics seems to be fading rapidly in Congress, the committees seem to be proceeding--in lieu of any better ideas-- to refashion the federal tax system in accordance with its precepts.

A sales tax can be made less regressive by exempting some of the daily necessities such as groceries. But there is no way that it can be increased, like an income tax, at the upper end of the income scale. As Congress edges away from the graduated income tax toward other kinds of taxation, it inevitably shifts the distribution of the burden downward.

The summer's tax cut was too big, as most of the people in Congress now recognize, and it is generating the prospect of intolerable budget deficits. There is the unhappy possibility that Congress will allow itself to be stampeded by the looming deficits into a flat tax of the sort about which Sens. Dole and Baker have been murmuring. The question is not only what gets enhanced here, but whom, and how much. As Congress reluctantly approaches the coming tax increase, it might remember that the best solution is to go back where it started, to the income tax--the fairest, most carefully considered, most finely balanced of all the American taxes.