Stephanopoulos said the selection process for a Supreme Court nominee will likely be headed by White House counsel Greg Craig, but that Vice President Joe Biden, who served on the Senate Judiciary Committee during six Supreme Court confirmations, could also play a big role.
President George H.W. Bush appointed Souter to the court in 1990. Souter came out of nowhere, a blank slate, unknown outside his home state of New Hampshire.
Bush had been assured that the quiet New Englander would be a solid conservative.
"If it were possible for me to express to you the realization that I have of the honor which the president has just done me, I would try, and I would keep you here as long tonight as I had to do to get it out," Souter said of his nomination.
But Souter was a surprise, and to conservatives, a mistake. He became one of the court's most reliable liberals.
"Justice Souter was somebody who came into every case with an open mind," said Feder, his former clerk.
Feder recalled several of Souter's most memorable opinions that defined his place on the court.
"What will be most remembered," Feder told ABC News, is his role in the 1992 decision that reaffirmed Roe v. Wade.
"At the time, most people expected that the recent appointments to the court would result in Roe being gutted or overruled. He wrote the part of the opinion stressing how important it was for court to respect prior decisions and not let major constitutional principles swing back and forth with every new appointment," Feder said. "Aside from the obvious importance of the opinion, it captures a fundamental part of his approach to judging."
Noting that Souter is a "deeply religious person himself," Feder said he "regularly voted to keep government out of religious matters" such as school prayer or the creation of a special school district for a religious sect.
"And Bush v. Gore has to be mentioned," Feder added. "He dissented from the decision to stop the Florida recount, and it's widely understood that he was deeply disillusioned by what the court did there."
An eccentric bachelor with a spartan lifestyle, Souter disliked Washington and for years has told friends he longed to go home. And now it appears he will get his wish.
ABC News Senior White House Correspondent Jake Tapper contributed to this report.