Justice Kennedy to play a new role

WASHINGTON — Justice Anthony Kennedy, who already decides whether liberals or conservatives win the Supreme Court's most closely contested cases, is about to take on an even more influential behind-the-scenes role with the retirement of Justice John Paul Stevens.

By virtue of seniority, Kennedy will inherit Stevens' power to choose the author of some court opinions, an authority that has historically been used — including in as big a case as the landmark Roe v. Wade abortion decision — to subtly shape a ruling or preserve a tenuous majority.

This change might keep the court's most liberal justices from writing some of its biggest decisions.

An unwritten high court rule gives the senior justice in the majority, most often the chief justice, the power to assign opinions.

When the liberals win an ideologically driven case by a 5-4 vote, the court's two senior justices — Chief Justice John Roberts and Justice Antonin Scalia, both conservatives — are sure to be on the losing side. With Stevens gone, Kennedy now is next in line.

The overall balance of power on the court is unlikely to change, with President Obama's choice of Elena Kagan to replace the liberal-leaning Stevens.

David Garrow, a Cambridge University historian who has written about the court, said the 74-year-old Kennedy already writes a disproportionate share of the court's big decisions and will have even more chances to do so now because he can assign opinions to himself.