WASHINGTON – If confirmed, Samuel Alito Jr. will sit on the first majority-Catholic Supreme Court in American history. And he will have Protestants and a liberal Catholic former Supreme Court justice to thank for it.

According to historians and conservative activists, the virulent anti-Catholic sentiment that animated much of American history began to erode quickly after the 1973 abortion case Roe v. Wade. Interfaith efforts to reverse that decision, among others, not only drove Catholics into the Republican fold, they say, but brought Protestants and Catholics together in a common cause.

In an ironic twist, the strongest advocate of the kind of judicial activism that conservatives say resulted in the Roe decision is also one of the most prominent Catholics ever to have sat on the court, William Brennan. The fact that Catholics and conservative Protestants have embraced Judge Alito in an effort to reverse Justice Brennan’s decisions, historians say, is the strongest evidence yet that Protestant suspicions of Catholic fealty to Rome were largely unfounded.

“This began as a marriage of convenience in the culture wars, but it has blossomed into an unprecedented spiritual fellowship,” a professor at Princeton University, Robert George, said. “If anyone is expressing concerns about Catholics being a majority on the court, it’s not backwoods evangelicals. It’s enlightened liberals.”

The Catholic and Protestant coalition that many credit with driving judicial activism to the center of the American political debate had its activist roots in a group that was formed in 1994 called “Catholics and Evangelicals Together.” The group was initiated by the Prison Fellowship founder and former aide to President Nixon, Charles Colson; and a Catholic priest and the editor of the ecumenical journal First Things, the Rev. Richard John Neuhaus.

The debate that grew out of this group’s discussions on the court led to a widely read series of essays on judicial activism by prominent Catholics, Protestants, and Jews. One contributor, the editor of the conservative magazine The Weekly Standard, William Kristol, who is Jewish, was instrumental in derailing the Supreme Court nomination of Harriet Miers. Mr. Kristol, along with evangelical and conservative Catholic activists, embraced Judge Alito.

“If Alito is confirmed there will be five Catholics on the court not because they are Catholic, but because they are originalists, textualists, or whatever term you want to use,” the Rev. Neuhaus said. “It will not be because they are Catholic, but because of their judicial philosophy.”

If rapprochement among Protestants and Catholics following Roe v. Wade helped lead to the current configuration of the court, then the Second Vatican Council’s teachings on openness toward other religions also played a part. A central theme of the Council, religious freedom, was developed by an American Catholic Jesuit, John Courtney Murray.

“After Vatican Two, we stopped calling them schismatics and heretics,” Mr. George said, “and we started seeing them as separated brothers who are not simply to be commanded to get back into the church, but as people who are to be talked to and listened to and argued with. What Murray brought was an American understanding and sensibility to a debate that was very much in need of it.”

The coalition between conservative Catholics and Protestants over the reach of the courts was also aided by the growing divide among Catholics on the issue. As Catholics sparred over the role that Rome should play in public life, many Protestants came to view the church’s members as capable of open disagreement. A recent sign that animosity toward Catholics had dissipated was the attendance of President Bush at the funeral of Pope John Paul II.

“Fifty years ago, an American president going to honor a pope in Rome was inconceivable,” the Rev. Neuhaus said. “It would have caused an enormous public ruckus – so also the prospect of having a majority of Catholics on the Supreme Court. Many things have happened since then.”

If the issues of abortion, homosexual marriage, and religious freedom were occasions for unity among conservative Catholics and Protestants, then they were equally potent sources of division among liberal and conservative Catholics. It was this division that Republican strategists exploited during the Reagan Revolution and which continues to define the vote-getting strategies of the Republican Party.

A major moment in the positioning of liberal Catholics came in a 1984 speech delivered at Notre Dame University by Governor Cuomo, of New York. Mr. Cuomo argued that his personal views about abortion should not be imposed on others as a matter of policy. Catholic politicians to this day refer to this speech when they speak of their personal opposition but public support for practices that Catholic moral teaching forbids.

“Judge Alito appears to be a competent lawyer and an expert judge,” Mr. Cuomo said in a phone interview with The New York Sun yesterday. “But the remaining question is, ‘Will he be true to the oath? The oath that is taken, as we know, is to make the Constitution the supreme law of the land, putting aside all personal opinions and even your own conscience when your own conscience is contradicted by the Constitution.”

An irony of the coalition that has led to the interfaith embrace of Judge Alito is that both he and Justice Brennan are natives of Newark, N.J., and that both are the children of immigrant Catholic parents. But beyond that, legal scholars say, the similarities between the two men end.

“When you talk about the Constitution as a living document, what you’re really saying is that it allows judges to adjust the law to contemporary conditions,” a former dean of the Catholic University Law School, Robert Destro, said. “Justice Brennan made it clear that, as a justice, he should embody the community that carries the law forward to make it reflect those aspirations.”

Justice Brennan was a recess appointment by a Republican, President Eisenhower, who thought he could win solidly Democratic East Coast Catholics in a second White House run. Some commentators have suggested that President Bush made a calculation to win Catholic support for Republicans with the nomination of Judge Alito.

American presidents nominated more than 50 justices to the Supreme Court before a Catholic sat on the bench. The first, Roger Taney, served during one of the most openly anti-Catholic periods in American history. The so-called “Know Nothing Party,” which formed largely out of worries over a new wave of Catholic immigrants in the 1840s, had 43 members in Congress in 1853.

Catholics held what was known as “the Catholic seat” on the court for much of the last century, until the confirmation of Justices Scalia and Kennedy in 1986 drove their membership up to two. Mr. Cuomo, who was thought to be a Supreme Court candidate during the Clinton administration, offered a different view of how Catholics arrived at their current prominence on the court.

“Catholics for years have been the sons and daughters of giants who came here with nothing and very early on went into government jobs because they didn’t have the wherewithal to go into big business,” he said. “Maybe what’s happened is a lot of them went into public service and became judges and worked their way up to the Supreme Court.”