The departure will open the first seat for a Democratic president to fill in 15 years and could prove a test of Mr. Obama’s plans for reshaping the nation’s judiciary. Confirmation battles for the Supreme Court in recent years have proved to be intensely partisan and divisive moments in Washington, but Mr. Obama has more leeway than his predecessors because his party holds such a strong majority in the Senate.

Replacing Justice Souter with a liberal would not change the basic makeup of the court, where he and three other justices hold down the left wing against a conservative caucus of four justices. Justice Anthony Kennedy, a moderate Republican appointee, often provides the swing vote that controls important decisions.

While the White House Counsel’s office prepared privately to step up its efforts to search for a replacement, Mr. Obama said through a spokesman on Friday that he had no direct knowledge of Mr. Souter’s plans.

“The President has not received a formal communication from Justice Souter,” Bill Burton, a White House spokesman, said Friday morning, “and he deserves the right to make his own announcement.”