Tom Maguire suggests that the National Journal finding that John Kerry is the most liberal member of the Senate isn’t supported by Poole and Rosenthal’s NOMINATE scores, at least not over the last two Congresses. He also quibbles:

Any fool can ask a question that ten wise men cannot answer: Dr. Poole bases his rankings on all recorded roll call votes, including the straight party-line organizational votes – for example, all Republicans voted for Bill Frist as Leader, and for the various Republican committee chairpersons. My suspicion is that the results give a good ranking within parties (so Kerry is really a centrist Dem), but the border between Republican and Democrat on substantive votes is blurrier than these results suggest. Objectivity and simplicity might suffer, but has this been looked at?

My (admittedly fuzzy) recollection of NOMINATE is that the results are fairly robust when you exclude pure party-line votes from the input data. A second approach to this question is a recent paper (released Monday!) by Joshua Clinton, Simon Jackman, and Doug Rivers that uses a Bayesian item-response theory model to approach the question (the same method used in their forthcoming APSR piece, a variant of which I used to measure political knowledge in my dissertation); the abstract follows:

We reanalyze the 62 key Senate roll calls of 2003, as identified by National Journal, using a statistical procedure that (1) is sensitive to different rates of abstention across senators and roll calls; (2) allows us to compute margins of errors on voting scores and the ranks of the legislators, as well as compute the probability that a given senator occupies a particular rank (e.g., is the “most liberal” senator). The three Democratic senators running for president in 2003 have markedly higher rates of abstention than the rest of the Senate, leading to considerable uncertainty as to their voting score (particularly for Senator Kerry). In turn, we find that contrary to recent media reports, Senator Kerry (D-MA) is not the “most liberal” senator, or at least not unambiguously; as many as three Senators could plausibly be considered the “most liberal“, with Kerry third on this list behind Senators Reed (D-RI) and Sarbanes (D-MD).

The note lacks any high-powered math, and should be accessible to anyone with an interest in politics and a modicum of statistical knowledge. Incidentally, their method does show a closer overlap between Democrats and Republicans than NOMINATE does (in part because they restricted the analysis to 62 “key” votes rather than all of the roll calls). One other thing to note: the whopping error bar around Kerry’s position, a direct result of his absenteeism from the Senate over the past year.