Is Sen. John F. Kerry a liberal? As the presidential campaign unfolds over the next seven months, the parties will no doubt spend a lot of time debating this question, with Republicans insisting that he is and Democrats just as vehemently denying it.

The question of how to measure a senator's or representative's ideology is one that political scientists regularly need to answer. For more than 30 years, the standard method for gauging ideology has been to use the annual ratings of lawmakers' votes by various interest groups, notably the Americans for Democratic Action (ADA) and the American Conservative Union (ACU).

The ADA, which describes itself as "the nation's oldest independent liberal organization," was founded in 1947 by a group of distinguished postwar liberals -- including Eleanor Roosevelt, labor leader Walter Reuther and historian Arthur Schlesinger -- to rally support for progressive causes. Shortly afterward, the ADA began publishing an annual legislative score card. Every year, the ADA's Legislative Committee selects what it considers to be the 20 most important votes cast in each house of Congress. Senators and representatives then receive a score ranging from 0 to 100, based on the percentage of times they voted for the liberal position, as identified by the ADA. In 1971, a group called the American Conservative Union began publishing a conservative counterpart to the ADA ratings, using the same method.

The ADA and ACU ratings are valuable as yardsticks for several reasons. Both have been around for a long time, thus providing some historical perspective. Both groups are able to speak with some authority about what constitutes the "liberal" and "conservative" positions on various issues. And both are good at distinguishing between meaningful and unimportant votes. Voters back home might be taken in if the House passes a resolution saying that all Americans have the right to adequate health care or a strong national defense -- but doesn't take any action or provide any money toward that goal. The ADA and ACU almost certainly won't.

So what do the ADA and ACU ratings tell us about Kerry? Here are the numbers for the past 10 of his 19 years in the Senate:













Kerry's 2003 ADA score may be a bit misleading. The ADA gives each senator five points every time he or she casts a liberal vote. Senators get zero points if they vote for the conservative position or if they don't vote at all. Of the 20 votes selected by the ADA in 2003, Kerry was absent for three. He thus actually voted the liberal position on all 17 of the votes he was present for.

Either way, Kerry's voting record is a very liberal one, according to both rating systems. The ADA's Web site notes that "those Members of Congress considered to be Moderates generally score between 40% and 60%." By that criterion, Kerry's record falls well outside the "moderate" range.

The same point is borne out by a comparison of Kerry's ratings with those of other Democrats who are often classified as moderates, such as Sen. John Breaux of Louisiana. Breaux's lifetime average ADA score through 2002 is 55. When Lloyd Bentsen of Texas was a senator, his lifetime ADA score was 41. Former Georgia senator Sam Nunn had a lifetime ADA average of 37. Al Gore had a 65 average. Joe Lieberman, who is sometimes described as a liberal and sometimes as a moderate -- he has a generally liberal voting record but also dissents from several important liberal positions -- has a lifetime ADA score of 76 through 2002.

At the other end of the spectrum, three senators are often singled out as the most liberal: Barbara Boxer of California, Pat Leahy of Vermont and Ted Kennedy of Massachusetts. Their lifetime ADA scores through 2002 are, respectively, 96, 93 and 90 -- statistically indistinguishable from Kerry's.

In recent weeks, a number of commentators have asserted that Kerry's voting history is complicated to classify. The evidence doesn't bear this out. If you were to take the numbers shown here, cover up Kerry's name and then ask a sample of American political scientists, "I have here a senator who in the past 10 years has had an average ADA score of 92 and an average ACU score of 6. Is he a liberal, a moderate or a conservative?" they would have no difficulty in classifying the 2004 Democratic candidate as, for better or worse, a liberal.

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William Mayer is an associate professor of political science at Northeastern University in Boston.