Dear Congressman: about that tax break. . .

While much of the country has long since forgotten, some in the hinterlands of America are outraged still over the tax break Congress gave itself late last year. Consider these scorching letters from the mail sack of US Rep. Eugene V. Atkinson (R) of Pennsylvania:

''There are not enough words to express the contempt I feel for the sneaky way the members of Congress gave themselves a tax break,'' writes one disgruntled constituent.

A married couple dryly proclaims that ''the buck stops here'' is an appropriate motto for Congress. ''Unfortunately too many legislators took that phrase literally,'' they write.

''Greed takes its eternal toll,'' say words penciled into the margin of a newspaper clipping and sent to the representative.

Several of the writers are incensed that their legislators are talking about trimming social security only months after giving themselves the right to deduct virtually all of their living expenses while they are in Washington. For many that means close to $20,000 in tax-free income - House members are eligible for deductions of $19,650 and senators can deduct $19,200.

Moreover, the measure was attached to a seemingly routine miners' benefit bill and not publicized until it had already passed.

Raising voters' hackles even higher, the Internal Revenue Service (IRS) has written a set of regulations sparing members of Congress from being audited on the new deduction. The IRS rules permit them to take a flat $65-a-day deduction for every ''legislative day'' of the year, including most weekends, going back to Jan. 1 of last year.

In Colorado, Texas, and Pennsylvania, the public has neither forgotten nor forgiven. With the help of reminders from radio talk shows, newspaper editorials , and a campaign sponsored by Common Cause, the self-proclaimed citizen's lobby, they are burying Washington with protest mail.

Representative Atkinson, a Republican, reports that he has received 600 letters on the issue, all with the same message. Some 6,000 comments arrived at the IRS, more than any new regulation has ever touched off, says a spokesman, and Common Cause this week delivered 18,000 to 20,000 more written protests.

Some members of Congress have hotly defended the tax deductions as fair and reasonable, arguing that the $62,500 annual congressional salary has fallen far behind inflation rates. The back-door method is the only way members will ever get a raise, according to the reasoning of some on Capitol Hill, because members will not go on record as voting themselves a raise.

The tax break protest has produced a flurry of activity, but no real action in the House, where all 435 seats are up for re-election this year. The hopper is filled with nearly 50 bills to repeal the measure, and no fewer than four ''discharge petitions'' have been filed to bring the issue to an immediate vote.

But a discharge petition needs 218 member signatures, and the most successful petition had 63 as of this writing. That is in spite of the fact that 150 members have sponsored bills to kill the tax break.

However, when the IRS held a hearing this week, only three members, Representatives Atkinson and Doug Walgren (D), also of Pennsylvania, and Berkley Bedell (R) of Iowa, showed up to ask that the IRS toughen the tax break rules.

''We are not staying at $75-a-night hotels or eating out in restaurants every night,'' Congressman Walgren told the hearing.

If a member eats out, said Walgren, ''believe me, it is generally at someone else's expense.''

Atkinson charged that Congress used an ''unethical, clandestine manuever'' to attach the tax break to a ''noncontroversial bill'' and that most members did not even know what the amendment was when they voted for it.

The IRS reports it will issue final rules on the tax deductions later this year. ''It is more typical that regulations are changed after a public hearings, '' says Jason R. Felton of the legislation and regulations division.

But Congress could still revoke the deductions, which previous law had limited to only $3,000. In the Senate, Sen. Russell B. Long (D) of Louisiana is the latest to join the crusade by filing a bill to revoke the measure.