To "Doonesbury" cartoonist Garry Trudeau, she may be the mildly distracted liberal Lacey Davenport. To some of her colleagues, she may be the little old lady who spends so much time on the floor of the House that she neglects her committee work. Her detractors may see her as a rich old lady whose efforts on behalf of the poor are of questionable sincerity. But nobody questions Millicent Fenwick's seriousness when it comes to congressional ethics. No one who knows her, for instance, would be surprised at her response to the special tax breaks members of Congress approved for themselves last winter.

"I refuse to take the special tax deductions," she said last week. "I did not take it this year, nor will I ever claim deductions that were established in such an entirely underhanded manner." The 72-year-old New Jersey Republican was referring to the packet of breaks the House sneaked into law last December, including a special expense deduction of $75 a day for every day Congress is in session, with no requirement for documentation. The measure was buried in a bill to restructure the black-lung benefit program.

"Absolutely outrageous," Fenwick declared. "This provision allows a member to deduct $19,650 without substantiating a single penny." She was also aghast at the way other congressional breaks were enacted: a special deduction for the cost of maintaining Washington homes and a provision for doubling (to $18,198 a year) the amount of outside income members are allowed to earn. Here's how that one came to pass:

Rep. John P. Murtha (D-Pa.) asked permission to introduce a resolution for "immediate consideration." The clerk started to read the resolution: "That clause 1 of Rule XLVII of the Rules of the House of Representatives . . ." That's as far as he got before Murtha interrupted to"ask unanimous consent that the resolution be considered as read and printed in the Record." Few members knew what the resolution was about, but, since there was no objection, it was adopted. The whole thing took about 20 seconds.

"I was sitting there in the House," Fenwick recalled later, "when Murtha rose to his feet and the speaker recognized him, and, bang, bang, bang . . . it was finished. It happened in seconds, and I didn't have any idea of what it was about. It was absolutely maddening. This is the pickpocket's way."

"It was the modus operandi, the subterfuge involved, that generated the public's angry outcry," she said. "Congress consistently has used this way of raising salaries and increasing financial benefits. A change is well overdue." The change she has in mind is contained in legislation she has introduced to prohibit automatic salary increases and to require a separate, recorded vote for every bill raising members' financial benefits. Her proposals would also delay the effect of such benefits to the next Congress, "so that each member of Congress first can stand for re-election," leaving it to "the taxpaying voter, not the member, to judge whether his or her representative deserves a raise."

Fenwick, now running for the Senate seat vacated by Harrison Williams, is also involved in efforts to repeal the special tax breaks that she has vowed not to use. "What the public can do," she says, "is to write to House Speaker Tip O'Neill and Senate Majority Leader Howard Baker to urge them to take action. Tell them to eliminate the tax deductions for living expenses and to reform the system so this deplorable business can't happen again." That may be vintage Lacey Davenport, but it's ethically solid.