Are Americans cheating more than they used to on their federal income tax returns?

That prospect sends shudders through the ranks of Internal Revenue Service officials here.

As authorities are fond of stressing, more than that of any other major nation, the American tax system has succeeded largely as a voluntary venture.

Compared to those in most other countries, enforcement efforts are minimal. Although most taxpayers still pay mainly out of fear of getting caught, historically the United States has had a fairly low delinquency rate.

As it stands now, the IRS audits only a little more than 2 percent of U.S. tax returns. The majority of taxpayers traditionally have paid up in full, wihtout major prodding.

Much as IRS officials don't like to admit it, however, there's growing evidence now that the situation may be changing -- rapidly.

First there are the widespread indications that the U.S. is developing an "underground economy" -- built around moonlighters and others such as waiters and cab drivers who simply don't report part or all of their income.

Although experts disagree widely over the size of this underground testimony, estimates are that the tax loss could be running into tens of billions of dollars. What's more, sociologists say the practice is on the rise.

Last week, a new General Accounting Office study showed that at least 8 percent of those required to file income tax returns in 1972, never did so -- a startling figure by any measure. And officials say that figure may be low.

Meanwhile, a Roper Poll published just a few days ago shows widespread cheating on federal income tax returns -- a factor some analysts take as bolstering suspicions that the practice is widespread.

Without knowing the results of the GAO study, which was not published until later, Roper's respondents estimated the proportion of non-filers at 7 percent of those eligible -- only a percentage point below the GAO's guess.

Those familiar with these developments give two major reasons for the apparent increase in tax cheating: First, inflation has crimped family income more than usual. And second, taxpayers are resentful of the size of the tax burden.

There's also more opportunity for cheating, if only because job patterns have changed so much. Today jobs are so plentiful that it's easy for a taxpayer to get part-time work in which tips or fees aren't recorded.

On its face, the problem still doesn't seem quite that serious. Experts say most of those who fail to file a return do so the first time as a harmless protest -- and then are afraid they'll get caught if they try to file again.

A probe by the congressional Joint Committee on Taxation in 1974 showed that some 50 of the 800 or so persons on the Nixon administration's "enemies" list -- those audited for political reasons -- failed to file returns.

What's more, at least at the surface level, the cost to the government isn't large. Many of those who fail to file don't own extra taxes, but actually are due refunds. Only in a few cases is the tax take worth pursuing.

Admittedly, the problem becomes far more serious if the estimates of the size of the so called underground economy are accurate. But even here, the bulk of the cases are bound to be small sized ones.

By far the major problem for the IRS is that perception of widespread tax cheating encourages still further violations and undermines the voluntary system -- raising the prospect of a need for more massive enforcement efforts.

The IRS already has a system for spotting and pursuing non-fliers and other tax cheats, and has streamlined it somewhat over the past few years. But officials concede the present effort needs beefing up. The question is how much is it worth spending to strengthen IRS enforcement efforts, and will Congress agree to vote the funds?

The Carter administration has been loathe to push for increased enforcement activities, if only to help hold down the budge deficit. Carter cut the IRS budget last year and for fiscal 1980, and Congress didn't object.

Now, however, there appears to be at least some move on Capitol Hill to provide more money for tax enforcement before the current trend gets out of hand and even greater efforts are needed.

The feeling before seemed to be that a little cheating now and then didn't hurt anyone. But now the practice is getting big.