CONGRESSIONAL ATTITUDES toward taxation grow more and more curious. Majorities in both Houses resist passing the full $25-billion tax cut that President Carter proposed, on grounds that it would be too inflationary. But simultaneously there is powerful and bipartisan support for a wide array of bills to open up new tax advantages costing billions of dollars a year, or reviving old ones. Most of those breaks would benefit mainly people with incomes well above the averages; some of the most popular would help the rich exclusively. It's obvious that there has been profound change in basic opinions about the tax structure - particularly among the Democrats.

During the Nixon and Ford administration, Congress enacted tax laws that made the federal income tax more progressive, and, by the definitions that most people accepted at the time, fairer. Now, with the White House occupied by a Democrat committed to tax reform, Congress with its large Democratic majorities is moving forcefully to make the income tax less progressive and perhaps unravel some of the last decade's reforms as well. The thread running through all of these bills seems to be a general feeling that the present tax structure taxes too much from the prosperous, to a point at which it hurts the national economy and everyone who depends on it. That idea was once the exclusive and undisputed property of the more rigorous among the Republicans. But no more. You could hardly have a sharper indication of the depth of the political changes now overtaking this country.

There is a lengthening list of tax bills illustrating the point. The votes on the tuition tax credit in the House yesterday suggest that President Carter is going to have to carry out his threat to veto it. There is the extremely expensive little bill to restore certain large benefits to Americans who work overseas. Above all, and most spectacularly, there is the Steiger amendment.

It would reduce substantially the tax rates on capital gains for people in high tax brackets, and it would take capital gains beyond the reach of the minimum-tax rule. The minimum tax was instituted to ensure that people with high incomes, regardless of their accountant's skill at constructing tax shelters, would have to pay at least a modest tax. The Steiger amendment would benefit at most some 400,000 taxpayers - that is to say, perhaps 8 percent of all those with taxable capital gains, and fewer than half of 1 percent of all those people who file income-tax returns. Nearly all of the benefits will go to people with incomes over $100,000 a year. The claim is that it will lift the stock market, encourage the rich to invest more and, through higher investment, eventually help everyone else. That line of argument is not a new one; the truly remarkable thing is that it is currently being embraced by a wide assortment of Democrats.

The Democratic leadership is sufficiently impressed to have begun serious negotiations over a compromise. It appears exceedingly likely that a least a toned-down version will become law this year.

The precise reasons for this astonishing swing in opinion are not yet fully clear. It has a lot to do with five years of high inflation. American attitudes toward money have been affected. Families have watched with dismay as their savings shrank, while the value of their property - especially houses - rose rapidly and became unexpectedly exposed to capital-gain taxation. Perhaps it also has something to do with the prospect of long years of relatively low economic growth ahead and poorer returns to investors than those to which they accustomed during the long boom of the 1960s and the go-go stock markets. Business investment has, in fact, been running lower than most economists think healthy.

The money involved in these little bills is less important than the evidence of change in American views of the rich, the poor, and the balance that represents that indispensable ingredient of tax policy, fairness.It is not a change in the direction of generosity or of the customary American ideals of a national community. But it is, apparently, genuine.