That was the case with Keeler. The cardinal, a former president of the U.S. Conference of Catholic Bishops, has built a legacy through his work in interfaith dialogue, particularly among Jewish groups.

"Keeler's retirement is going to be a major loss for ecumenism and for Catholic-Jewish relations," Reese said. "He's just someone that ... all the Jewish leaders are very comfortable dealing with."

For Vatican observers, what happens in Baltimore is being watched for any hints on changes to come in New York and Detroit, whose cardinals have also reached their 75th years.

Baltimore's new archbishop won't necessarily be a cardinal - Keeler waited five years before receiving the honor, and other cities have a stronger tradition of leaders with red hats. And with five of the 11 American cardinals now based in Northeastern dioceses, speculation suggests the pope will soon create a cardinal in the South or West, where Catholic populations are booming because of Latino immigration and Catholics relocating. "I don't think it's necessarily the case that 25 years from now, most of the American cardinals will be strung out along I-95," Weigel said.

Pope Benedict has appointed 26 bishops in the United States. His first was Bishop Dennis J. Madden, Baltimore's urban vicar.

Secretive process
The selection process is secretive, and the pope makes the ultimate decision. According to the American bishops' conference, the process of selecting a successor for a retiring bishop begins with a report on the state of the diocese, prepared by the incumbent.

The apostolic nuncio, who serves as the pope's envoy to both the national government and to the Catholics of the country, receives the report and investigates possible candidates by consulting with bishops and archbishops, particularly those who know the area. He develops a terna, or list of three names, as well as a report with his recommendations.

That information is sent to the Congregation for Bishops at the Vatican, which discusses the appointment and votes on the candidates. Then the prefect of the congregation brings all the information to the pope, who can pick from that list or ask for new candidates.

It's very difficult to identify trends among Pope Benedict's appointments, said Archbishop Thomas C. Kelly of Louisville, Ky., noting all the variables that go into the decisions - the needs of the community, the wishes of the apostolic nuncio and of the pope.

Reese said, "This is a situation where the people who know don't talk and the people who don't know, speculate," he said.

His research revealed that popes rarely move archbishops to another archdiocese - except in Chicago - and that usually appointments come from outside the diocese. Likely candidates would be about 65 years old - with sufficient time before their 75th birthday to lead the new see.

There's too little data to generalize about Pope Benedict's choices, but Reese said he has picked highly educated candidates. "He's looking for bishops who will be teachers to their flock," he said. "They seem to be less confrontational than people appointed under [Pope] John Paul II."

Palmo agreed, describing Pope Benedict's appointments as "people who come from the classroom but are still pastorally comfortable."

"Benedict appointees are not scared of dialogue," Palmo said.

For Baltimore, some candidates being mentioned in Catholic circles include two African-Americans: Ricard, 67, a former urban vicar of Baltimore who left a decade ago to lead the Pensacola-Tallahassee diocese; and Bishop J. Terry Steib, 66, of the Diocese of Memphis, who has had notable success with vocations.

Ricard is a member of the Josephites, a Baltimore-based religious order that ministers to African-Americans. Steib, of the Society of the Divine Word, was praised in 2005 newspaper accounts for tripling the number of seminarians, to 15.

Two former rectors of the Pontifical North American College, the American seminary at the Vatican, could also be considered - Archbishop Edwin F. O'Brien, 68, who leads Catholics in military services, and Dolan, 57, of Milwaukee, who preached at the Mass for the 50th anniversary of Keeler's ordination.

One possible replacement who could continue Keeler's interfaith work is Bishop William F. Murphy, 66, of the Diocese of Rockville Centre, which represents about 1.5 million Long Island Catholics. He has served on the federal Commission on International Religious Freedom and is a former chairman of the U.S. bishops' committee on ecumenical and interreligious affairs.

According to news reports, he banned Catholic lay group Voice of the Faithful from meeting on church property. The Massachusetts attorney general also investigated but did not pursue further action against Murphy, a former auxiliary bishop in Boston, in connection with that archdiocese's handling of clergy abuse there.