Thompson, a Republican and actor on the NBC television series "Law & Order," agreed to accept the post in a telephone conversation with the president on Monday, McClellan said.

He said Thompson would serve as an informal adviser to shepherd the nomination through the Senate.

"Senator Thompson will guide the nominee through the confirmation process," McClellan said.

Justice Sandra Day O'Connor surprised Washington last Friday by announcing her retirement from the high court after 24 years, giving Bush his first opportunity to name a new justice. Court watchers were expecting such an announcement from Chief Justice William H. Rehnquist, who is 80 and afflicted with thyroid cancer.

Thompson, 62, retired from the Senate in 2002 to pursue an acting career. He has appeared in the movies "The Hunt for Red October," "Cape Fear," and "In the Line of Fire."

He was elected to the Senate in 1994 to serve out the term of Democrat Al Gore, and was easily re-elected in 1996. He retired in 2002. Thompson was a member of the Judiciary Committee, which will hold confirmation hearings on the eventual nominee.

Senate Majority Leader Bill Frist, R-Tenn., said his former colleague has a "profound understanding" of judiciary matters and is known for a "no-nonsense style."

"Fred is a well-respected leader admired on both sides of the political aisle as someone who is fair-minded and straightforward," Frist said in a statement. "He is experienced in the ways of Washington and will undoubtedly provide valuable insight and counsel throughout the confirmation process."

Ed Gillespie, former Republican National Committee chairman, also will help the administration push Bush's nominee. He is expected to help with strategy, lobbying and news media relations.