Ideological Composition of the Supreme Court

Below we use the Martin-Quinn scores of judicial ideology to plot the ideological makeup of the Supreme Court. Political scientists Andrew Martin and Kevin Quinn use a technique similar to NOMINATE to estimate ideological ideal points from votes– roll calls for Congress and case decisions for the Supreme Court. This topic has recently gained salience with the Supreme Court’s announcement that it will hear a challenge to the constitutionality of President Obama’s health care reform law (the Patient Protection and Affordable Care Act), passed in March 2010.

We first show the ideal points for Justices of the current Court along the ideological (liberal – conservative) dimension. In accord with popular perception, the Court’s liberal bloc is composed of Justices Ginsburg, Breyer, Sotomayor, and Kagan (all appointed by Democratic presidents– Clinton and Obama); while the Court’s conservative bloc is made up of Justice Thomas (by far the most conservative) and Justices Scalia, Roberts, and Alito (all of which were likewise appointed by Republican presidents– Regan and George H.W. and George W. Bush). Justice Kennedy also leans to the right, but is the median justice on the Roberts Court and thus whose decision on which the Court’s decision is likely to hinge.

Because the debate over whether the health care law’s individual mandate (which requires individuals to purchase health care insurance) is a constitutional exercise of the federal government’s powers under the Commerce Clause, we also plot the ideological composition of the Rehnquist Court in 1996. In that year, the Court reached a major decision on a case (U.S. v. Lopez) involving whether the Commerce Clause applied to the Gun Free School Zones Act of 1990, a law which prohibited carrying firearms into public schools. In this case, the Court ruled 5-4 that the law was unconstitutional as an illegitimate extension of Commerce Clause powers. From the standpoint of the spatial (geometric) model of voting, the cut-point is located between Kennedy and the liberal bloc, with perfect classification (that is, all members to the left [liberal side] of the cutpoint voted alike, while all members to the right [conservative side] also voted in unison on the opposite side. This gives at least some indication that divides over the interpretation of federal powers allotted by the Commerce Clause track the ideological dimension. With the health care case, the question is whether the cutpoint will fall to the left or right of Justice Kennedy.

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