President Gerald Ford and Cardinal William Baum, then archbishop of Washington, in 1976. (Ken Feil/The Washington Post)

William W. Baum, who, as the Catholic archbishop of Washington, welcomed Pope John Paul II to the nation’s capital in 1979 and who became the longest-serving U.S. cardinal in history, died July 23 at a Catholic care facility in the District. He was 88.

His death was announced by Cardinal Donald Wuerl, archbishop of Washington. The cause of death was not disclosed.

Cardinal Baum was archbishop of Washington from 1973 to 1980, during a time of questioning and inner turmoil in the Catholic Church. He was known for initiating dialogue with members of other faiths and for expanding the church’s outreach efforts among minorities.

He was named a cardinal in 1976 by Pope Paul VI and participated in papal conclaves that elected three popes, including two who were close friends: Pope John Paul II and Pope Benedict XVI.

Cardinal Baum’s 39 years as a cardinal marked the longest tenure of any American Catholic prelate in history, surpassing the 35 years served by James Gibbons, an archbishop of Baltimore, who died in 1921.

With a Protestant father and a Jewish stepfather, Cardinal Baum embodied an ecumenical outlook from birth. In his 30s, he was already recognized as one of the Catholic Church’s leading authorities on interfaith relations.

During the Second Vatican Council of the 1960s, when the church was reassessing its role and teachings in a rapidly changing world, Cardinal Baum served as a principal adviser on ecumenical matters to church leaders. He drafted some of the documents on ecumenism for Vatican II.

Throughout his career, he was considered an intellectual force within the church and held powerful positions at the Vatican, where he spent almost as much time as in his native country. He spoke fluent Italian.

“As a place of exile,” he once said, “Rome is not so bad.”

Cardinal Baum lived in Washington from 1964 to 1967, when he was the first executive director of the Bishops’ Committee for Ecumenical and Interreligious Affairs. After serving as a bishop in Missouri, he returned to Washington in 1973, succeeding Patrick O’Boyle as archbishop. At the time, the archdiocese was home to about 390,000 Catholics.

In his first homily as archbishop, Cardinal Baum declared that the church “clearly must do battle with racism and with all other forces which threaten human life and liberty.”

He described racial prejudice as a “heresy and sin.” In 1974, he established a Black Secretariat, or a council of clergy members and lay people designed to give African Americans a greater voice in the church.

Cardinal Baum became archbishop the same year as the U.S. Supreme Court ruled in Roe v. Wade , a decision that legalized abortion nationwide. In keeping with the church’s position, the cardinal remained an outspoken opponent of abortion.

“We do not see our efforts on behalf of human dignity as the imposition of our religious beliefs on others,” he said in 1976.

Although he was known as a conservative in matters of church doctrine, Cardinal Baum’s approach was one of pragmatic sympathy rather than confrontation. He adopted a motto drawn from St. Paul’s letter to the Corinthians: “Ministerium reconciliationis,” or the ministry of reconciliation.

The only event during his seven years as archbishop that was remotely a scandal was the archdiocese’s 1974 purchase of a $525,000 mansion as a residence for Cardinal Baum. The backlash was highlighted by a 25-day hunger strike by dissident priest Edward Guinan, who said the church should spend its money on the poor.

Cardinal Baum rescinded the purchase of the house and continued living at the rectory of the Cathedral of St. Matthew the Apostle in Northwest Washington.

“As a bishop and a pastor,” he told The Washington Post in 1974, “it seems to me that I’m engaged in . . . how to conserve and unite, heal and nurture and sustain the gospel with all its radical demands. Isn’t this always a task for the Church?”

William Wakefield White was born Nov. 21, 1926, in Dallas. His father was Presbyterian, but his Catholic mother steered him toward her religion.

He later moved with his mother to Kansas City, Mo., and was tight-lipped about whether his father died or his parents were divorced. He later adopted the last name of his Jewish stepfather, Jerome Baum, who died when William was 12.

As a child, the future priest went to some Presbyterian services, later telling The Post, “No one ever suggested to me that attending Protestant worship was wrong.”

But by age 12, he was going every day to Catholic Mass, and he entered seminary training at 13.

He was a graduate of what is now Kenrick-Glennon Seminary outside St. Louis and was ordained a priest in 1951. He was a college teacher and a priest in Kansas City before spending two years in Rome, where he received a doctorate in theology in 1958 from the Pontifical University of St. Thomas Aquinas.

After his advisory role during Vatican II and his ecumenical leadership post in Washington, Cardinal Baum served as a parish priest in Kansas City. He was named bishop of the Springfield-Cape Girardeau diocese in Missouri in 1970.

In 1971, as a delegate to a Vatican gathering of bishops, he helped write a memorandum calling for the church to oppose the “ongoing armaments race among the powerful nations of the world.”

A year after Pope John Paul II’s 1979 visit to Washington, Cardinal Baum was assigned a new role at the Vatican, in charge of the church’s educational policies and regulations for Catholic schools, colleges and seminaries.

He was the highest-ranking American in the Curia, or the administrative arm of the Vatican. In the 1990s, he served on a commission that drafted a new Catholic catechism.

From 1990 until his retirement in 2001, Cardinal Baum held the office of major penitentiary, in which he helped resolve confidential issues of conscience and procedural matters within the church.

He lived for years in a book-lined apartment overlooking St. Peter’s Square and was known as a knowledgeable lover of opera and classical music. Cardinal Baum moved back to Washington in 2006.

Asked to explain his record-setting­ tenure as America’s longest-serving cardinal, he said only, “It’s a gift from God.”