(FORTUNE Magazine) – Who is the most liberal member of the U.S. Senate? The last time we asked this fateful question (June 19, 1989), the surprising answer was Claiborne Pell, the spaced-out aristocrat from Rhode Island who heads the Foreign Relations Committee and is the only member of the upper chamber who could possibly have said ''Do tell Mr. McAn that I'm much obliged'' when an aide bought him a pair of galoshes and mentioned that they were from Thom McAn. Pell did not repeat this year, but his successor also proved to be an astonishment. Or at least the winner was not among the more obvious suspects: Teddy Kennedy of Massachusetts, Paul Simon of Illinois, Howard Metzenbaum of Ohio, Tom Harkin of Iowa, Alan Cranston of California, Paul Sarbanes of Maryland. Who can it be? Our procedure for ferreting out the furtive fanatic was, as in the past, to begin with rankings provided annually by the three main liberal pressure groups: Americans for Democratic Action (ADA), the AFL-CIO, and the American Civil Liberties Union (ACLU). All three tell you which congressional votes they deem critical and how each member voted. These votes are converted into scores ranging from 0 to 100, i.e., a ''perfect liberal'' would score a total of 300. Pell won with 277, but this year's guy (that's right, it's also not obvious suspect Barbara Mikulski of Maryland) was even more dirigiste, weighing in at 287. Somewhat influenced by a yen to play with Q&A, a clever software package developed by Symantec Corp. of Cupertino, California, we computerized this year's search. Q&A's unique feature is something called the Intelligent Assistant, which enables you to converse with your database in ordinary English. In addition to each Senator's numerical rankings, our database incorporated a wealth of information on such arguably explanatory variables as age, political party, duration of Senate membership, mustache status, and aye- nay record for each of the 45 critical votes selected by one or another of the pressure groups. The data on presence or absence of a mustache (obtained by inspecting photographs in the Almanac of American Politics) were incorporated to test the hypothesis that liberals are more likely than conservatives to have hair on their lips. While ultimately quite fascinating, our senatorial mustache data proved to have less differentiating power than postulated. When we turned to Q&A and typed in ''List all the Senators who have mustaches,'' it sullenly responded, ''No forms were found that meet your retrieve request.'' (Okay, we never said it spoke Oxford English.) In other words, not one Senator has a mustache. This finding is puzzling, as we have since established that 38 members (8.7%) of the lower chamber have them. If an average Senator were as much predisposed to mustaches as is an average Congressman, then the chance of electing a Senate with zero mustaches would be only one in 9,330. Clearly, mustaches are viewed as uniquely unsenatorial, but why? That is the question. Back to Senator Biglib. We had a bit of trouble finding the right words for Q&A, but it finally understood the question when we typed in: ''Which Senator has the maximum when you add ADA score, ACLU score, and AFL-CIO score?'' The screen instantly displayed an amazing answer: ''Leahy -- 287.'' We had frankly never thought of Pat Leahy, the senior Senator from Vermont, as being superliberal, our main association to the chap being the 1987 episode in which (a) the Intelligence Committee voted not to release a certain report, (b) Pat nevertheless showed his copy to an NBC correspondent, (c) the copy was displayed on the air, (d) Pat offered his resignation from the committee, and (e) nobody tried to stop him. His ratings show him to have been less liberal in the past. Possibly he has been tilted leftward by the electoral . triumph of Vermont's sole Representative, avowed socialist Bernie Sanders, who got 56% of the state's vote in 1990. It takes a heap of horrible votes to become the Senate's biggest liberal. To do it, Leahy had to appease the ADA by voting for sanctions against Iraq (in effect a vote not to launch a military attack); mollify the AFL-CIO by voting for extension of the Davis-Bacon Act (requiring use of high-paid union labor on federal construction); and cuddle up to the ACLU by voting against an amendment that would have restricted the right of the National Endowment for the Arts to use public funds for works depicting ''excretory activities.'' At that, Pat was almost edged by Sarbanes, who had 286 points. The Marylander lost out when he could not bring himself to vote against an amendment (adopted 81 to 8), assailed by the ACLU, penalizing health care providers with AIDS who continue to perform invasive surgery without notifying patients. Leahy voted against it. He is lucky that Q&A does not editorialize.