JUST BEFORE CHRISTMAS, members of Congress voted themselves a big tax break. For good measure, they made it retroactive to January 1981. Just how big a break it is became apparent last week when the Internal Revenue Service published regulations that will guide this select group of citizens. What is not yet clear is whether the members of Congress will still be taxpayers.

This is how the new law works. Using the assumption that members of Congress, when they are in Washington, are really just like businessmen who are traveling away from home, the law allows them to deduct all expenses they incur in connection with their work in the capital. This includes food and housing in Washington, as well as myriad incidentals such as taxis, personal automobile travel, household help, laundry, utilities and home maintenance. A deduction is also allowed for depreciation of a Washington-area home and for the furnishings in that home.

Members of Congress can choose any one of three ways of computing this deduction:

First, they can take a flat $75 a day for every single day of the year except those periods during which Congress is in recess for five days or more. It is quite possible that congressional schedules will be adjusted and pro forma sessions added so that such recess periods will be substantially reduced in number. Assuming that a 30-day August recess is preserved and that Congress is in session from Jan. 15 to Dec. 15, the $75 deduction would be available for 305 days for a writeoff of $22,875.

This deduction is available even when the member is not actually in Washington, and could be used even for days when other deductible business expenses were incurred elsewhere--for example, while giving a speech in another state. If the member chooses this method of computing his taxes, he need not prove his expenditures and he cannot be challenged by IRS on this matter in an audit. Ordinary businessmen do not have this option.

Second, if the congressman has a big mortgage and high property taxes in the Washington area, he may choose to figure his allowance by deducting two-thirds of the amount described above plus his mortgage interest and property taxes. Thus someone with a $1,000-a-month mortgage interest payment and $3,000 in taxes would have an automatic deduction of $30,250.

Finally, the big spenders may choose to keep track of their expenditures and take an unlimited deduction so long as records are kept to prove their claims. Members with large homes, expensive furniture, lots of household help and big cars may find it to their advantage to use this method.

It is important that the public understand the magnitude of this tax break and be given an opportunity to monitor its use. Members of Congress who use the new law--and surely they all will--should voluntarily disclose the amount of this deduction.

If it appears after a year that these tax breaks, together with all other deductions and exemptions available to everyone, result in the virtual elimination of income taxes on congressional salaries--a fear expressed by anonymous IRS officials last week --the law should be changed. A salary increase is far preferable to tax concessions that would isolate members of Congress from this burden of citizenship. In the interest of all of us, tax writers should be tax payers.