Since President Obama’s nomination of Elena Kagan last week to succeed retiring Justice John Paul Stevens, there has been a lot of talk about whether she would be a persuasive force in moving Justice Anthony Kennedy to the left. I’ve felt that much of this discussion ignores the limits of a new, junior justice, and I think Monday’s decision in the juvenile sentencing case offers a reminder of Justice Kennedy’s own force.

Kennedy wrote the Court’s decision that declared for the first time that juvenile offenders cannot be locked up without a chance of parole for crimes that do not involve homicide. (Read stories on the ruling here and on Kennedy here.) The decision in Graham v. Florida demonstrated Kennedy’s continuing role as the crucial fifth vote on the Court, as well as his dominant voice on the Eighth Amendment, which prohibits cruel and unusual punishment.

Monday’s decision flowed from two earlier Kennedy opinions, the 2005 Roper v. Simmons, which forbade the death penalty for juvenile criminals, and the 2008 Kennedy v. Louisiana, which prohibited the ultimate punishment for the crime of child rape.

As Kennedy, joined by the Court’s four liberals, extended the reasoning of Roper v. Simmons beyond the realm of capital punishment Monday, he wrote, “It is true that a death sentence is unique in its severity and irrevocability, yet life without parole sentences share some characteristics with death sentences that are shared by no other sentences.” Life without a chance of parole, he said, “deprives the convict of the most basic liberties without giving hope.”

Chief Justice John Roberts tried to make the case – beginning with his remarks at last November’s oral argument and then in his separate opinion in Graham – for an individualized approach, rather than a blanket rule against life-without-parole. But Roberts could not persuade Kennedy. And while senior liberal Stevens can often be credited with convincing Kennedy to join those on the left, in this Eighth Amendment dispute over penalties for young offenders, Kennedy was – more likely than not — already there.