(FORTUNE Magazine) – In which Keeping Up returns to its traditional search for Senator Biglib, the most liberal member of the current upper chamber. Two years ago the menace to society turned out to be Democrat Pat Leahy of Vermont. As usual, we begin with the latest ratings supplied by three liberal groups: Americans for Democratic Action (ADA), the AFL-CIO's Committee on Political Education (COPE), and the American Civil Liberties Union (ACLU). This year we have a new wrinkle. We take the average of the three liberalism ratings and from it subtract the conservatism ratings of the American Conservative Union (ACU). Like its liberal counterparts, the ACU gives everyone a grade of 0 to 100. Our new scoring system kicks out positive figures for all ''net liberals,'' with Senator Biglib on top at 90.33. It results in negative figures for conservatives, led by Jesse Helms of North Carolina at -90.00. It also produced a score of 0.00 for Democrat Jim Exon of Nebraska, a super-middle-of-the-roader of whom the Almanac of American Politics correctly states, ''His voting record is just about at the Senate midpoint on economic, cultural, and foreign issues.'' The ranking of the 100 Senators turned up a rather dramatic display of party differences: The 50 most liberal Senators were all Democrats; the 38 least liberal were all Republicans. So now you know Biglib's party. Many folks believe ''liberal'' no longer has any core meaning, and further argue that the three liberal rating groups are concerned with different issues. The view is facially plausible. Of the many different votes cast by Senators in 1993, only one (on the Family and Medical Leave Bill) was reflected in the ratings of all three liberal groups. COPE has no position on the death penalty, and the ACLU has none on NAFTA. And yet -- there clearly is some kind of common denominator in the three sets of ratings. Proof of its existence lies in the correlations among the three. Positive correlations range from 0.0 to 1.0, and any figure close to 1.0 tells us that the relationships are quite strong. It turns out that ADA scores correlate 0.93 with those for COPE and 0.87 with those for the ACLU; COPE and the ACLU in turn correlate at 0.79. Bottom line: Senators who get high marks from any one of those groups are a good bet to please the other two as well. This year's Senator Biglib, with a 95 from ADA, a 96 from COPE, an 80 from the ACLU, and a goose egg from ACU, is Paul Sarbanes of Maryland. For an 18- year veteran of the Senate, Sarbanes is relatively obscure, perhaps because, according to the Almanac of American Politics, ''he actively disdains publicity.'' Ah, but if he wants to stay out of the Keeping Up spotlight, he'd better lay off the dirigisme. He gets groaned about here because of his vote for raising taxes by $243 billion over five years, for a huge, new public-works spending program to get the economy going, for shoring up the Davis-Bacon Act (which requires union wages on federal building projects), for rejecting NAFTA, and, of course, for the Family and Medical Leave Bill, loved by libs everywhere even though it leads to fewer jobs for young women who look as though they might get pregnant. With or without publicity, he made it look easy.