President Obama and other supporters of Elena Kagan have argued that she has the capacity to form coalitions with conservatives, especially Justice Anthony Kennedy -- the Supreme Court's lone swing vote. There are a number of underlying assumptions to this argument, including that Kennedy's opinions are malleable on a significant number of issues and that a colleague on the bench can push him on those malleable questions.
There is certainly some legitimacy to these assumptions. Political scientists who research the Court have found that ideological moderates are among the most malleable members of the bench. Furthermore, Sandra Day O'Connor and David Souter most likely influenced Kennedy in the influential case Planned Parenthood v. Casey, which upheld Roe v. Wade -- even as it opened the door to far more intrusive regulations of abortion. These observations, however, do not demonstrate that Kagan or any other future justice can wield influence upon Kennedy (or other conservatives).
First, it is unclear whether Kagan herself is a progressive or a political moderate, like Kennedy or O'Connor. Her academic writings just do not provide enough insight to place her definitively within a particular judicial camp.
Furthermore, supporters of the idea that Kagan can move Kennedy discount the substantial role that other factors play in shaping judicial opinion. The positions held by the Executive, Congress, social movements and voters all impact judicial decisionmaking, and according to the academic literature in this area, moderates are more susceptible to these external influences than others. Viewed in this light, Kennedy's vote to uphold Roe could reflect the fact that a majority of voters believe in the right to terminate a pregnancy. Similarly, his vote against "partial-birth" abortion could relate to the fact that a majority of voters oppose late-term abortion.
Of course, Kennedy's own ideology, Court precedent, the facts of each case, arguments of legal counsel, and debates with other justices likely influence Kennedy's opinions as well. But the assertion that Kagan can serve as a consensus builder fails to acknowledge the host of other factors outside of debates with colleagues that substantially impact judicial opinion.
People who believe that Obama should appoint someone who can "flip" Kennedy have a limited understanding of the dynamics of judicial decisionmaking. They reduce it to an intellectual exercise where the "best argument" combined with grace and warmth dictate outcomes. Also, as Dalia Lithwick argues, liberal advocates of a Kennedy pal affirm a myth that "conservative judges closely read the Constitution and apply the law, while liberals stick a finger in the wind and then work the room." Both camps, however, are motivated by ideology and external political factors. This reality makes the search for someone who can sway Kennedy a bizarre calculation for a nominee to the Supreme Court.
Note: Other legal commentators have made similar arguments. See:
Is Kennedy Easily Manipulated
Asking "Who can sway Kennedy?" is no way to pick Justice Stevens' replacement.