In the event of a US Supreme Court vacancy, would it be more difficult for President Obama to replace moderate conservative Anthony Kennedy or the more extreme conservative Antonin Scalia? Presuming that the president would nominate a solid liberal, it might seem that replacing Scalia would be more difficult, because replacing Scalia would cause a more leftward shift in the seat than replacing Kennedy would.
But my research here suggests that the president would receive less resistance from the Senate for a liberal-for-Scalia change. Here's why: the key senator required for confirmation is one of the more moderate senators -- the 51st senator for the confirmation vote, and the 60th senator for a cloture vote in case of a filibuster -- and the liberal-for-Scalia change is more attractive to these senators. Consider the figure below that presents the Bailey ideological ideal points of the nine Supreme Court justices in 2008 (in black) along with the ideal points of the Senate medians in 2006 and 2008.
The empirical evidence in the article suggests that senator opposition to Supreme Court nominees is a function of whether the change makes the senator better off ideologically in terms of the senator's ideology being reflected on the Court. Conservative senators are likely to oppose the nomination of a liberal in either case because both the liberal-for-Kennedy change and the liberal-for-Scalia change move the ideology of the seat away from conservative senators; liberal senators are likely to support the nomination of a liberal in either case because both the liberal-for-Kennedy change and the liberal-for-Scalia change would move the ideology of the vacant seat toward liberal senators; but there's a difference for moderate senators: the liberal-for-Kennedy change would move the ideology of the vacant seat away from the moderate senators close to Kennedy, but the liberal-for-Scalia change would not change for moderates, because neither Scalia nor the liberal nominee are close to the Senate median.