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.

David Karol at Monkey Cage discusses the Supreme Court:

The broader point is that the whims of one unaccountable person, whatever their age, abilities or ideology should NOT matter so much in a democracy.

Karol proposes eighteen-year terms, which reduce the length of time that the "whims of one unaccountable person" matter but do not reduce the relative importance of that person's whims on any given case or in any given year. But court expansion would reduce the "cross-sectional" power of a justice and thus reduce the associated concern for the health of like-minded justices, severely curtailing the chance of a judicial Weekend at Bernie's.

Of course, it is not politically feasible to give any president multiple new nominations, but it is possible to expand the Court to eighteen members without packing and with minimal disruption to the status quo:

1. Give each current justice two votes and each new justice one vote: when a current justice vacates a seat, the president and the Senate replace the current justice with two new justices; when a new justice vacates a seat, the president and the Senate replace the new justice with one new justice.
2. Let's say that we want an eighteen-member Court immediately. Let each sitting justice nominate one new justice. The sitting justices would have a strong incentive to nominate a candidate with similar judicial and ideological views.

Eighteen-member or eighteen-vote Courts would have a greater potential for tie votes, but this is less a problem than a feature with positive consequences, since a majority of the full Court is now a 10-to-8 supermajority, which makes it slightly more difficult for the Court to overrule a lower court or alter its own precedent.