Bishop David Zubik of the Catholic Diocese of Pittsburgh has joined a chorus of bishops calling opposition to abortion the paramount issue in Tuesday's election.

"Moral issues involving the sanctity of human life command pre-eminent consideration," he wrote in his column in Friday's Pittsburgh Catholic newspaper.

"The panorama of life issues include world poverty, warfare, capital punishment, cloning, embryonic stem-cell research, torture, human trafficking and the very institution of marriage among others," he wrote. "But pre-eminent among the life issues are abortion and euthanasia."

He endorsed a national call for Catholics to spend election eve in prayer vigils against a long list of societal evils.

Rocco Palmo, whose blog "Whispers in the Loggia" tracks the hierarchy, counted 71 of nearly 250 U.S. bishops, or about 30 percent, who have stressed abortion over other issues in this election. The others have remained silent or stressed other issues, he said.

Differences among bishops hinge on how they interpret the U.S. bishops' document, Faithful Citizenship, which calls opposition to abortion crucial, but names other issues that Catholics must consider. It says there are limited circumstances in which it could be permissible to vote for an abortion rights candidate.

"A Catholic cannot vote for a candidate who takes a position in favor of an intrinsic evil, such as abortion or racism, if the voter's intent is to support that position," the bishops wrote.

Because of the stark difference between Mr. McCain and Mr. Obama on abortion rights, statements that emphasize abortion as a voting issue stop just short of a McCain endorsement, said the Rev. Thomas Reese, senior fellow at Georgetown University's Woodstock Theological Center.

However, Father Reese believes it's legitimate to argue that voting for Mr. Obama will support social services that may prevent more abortions than would voting for a Republican and attempts to reverse Roe v. Wade.

"Some of the bishops are taking Faithful Citizenship and where it says that you could vote for the candidate who is pro-choice if there were other grave reasons to do so, they are basically saying that there are no other grave reasons," he said.

Helen Alvare, associate professor of law at George Mason University and a prominent anti-abortion activist, said bishops were right to stress abortion. She cited Mr. Obama's pledge in a speech to Planned Parenthood last year to sign the Freedom of Choice Act, which would eliminate current restrictions, mandate federal funding and require even Catholic hospitals to perform them.

"I'm in favor of generous social programs. But any program that fails to combine social support with a robust moral message that life is a gift, not a curse ... is unrealistic," she said

The Freedom of Choice Act is why Bishop Zubik said he can't agree that a vote for Mr. Obama would promote services that reduce abortions. But asked if a vote for Mr. Obama would be sinful, he cited voter intent.

"A person would have to have a grave moral reason," he said. "The issue of sin is between a person and God. The reality is that each of us knows what is our intention when we do whatever we do."

His intention in writing was "to clarify the pre-eminence of the abortion issue because I think a lot of people have been confused into thinking that the whole line of pro-life issues are equal. But they aren't."

Nevertheless, the Pittsburgh diocese forbids distribution of campaign-related literature on parish property, with the sole exception of the voters' guide produced by the Pennsylvania Catholic Conference. Auxiliary Bishop Paul Bradley wrote to pastors Friday reiterating that anyone who tries to distribute other literature "should be asked to leave the premises."

Bishop Zubik's column was in line with a statement issued last month by all of Pennsylvania's Catholic bishops.

"Catholic teaching does not treat all issues as morally equivalent. The protection of human life from conception until natural death is the preeminent obligation of a truly just society," it said.

Some bishops have objected to voter forums on church property in which speakers offered divergent views on Faithful Citizenship. Shortly before Douglas Kmiec, an abortion opponent who nevertheless endorsed Mr. Obama, was to speak Oct. 22 at Seton Hill University, Bishop Lawrence Brandt of Greensburg protested via diocesan Web site.

"Mr. Kmiec distorts Catholic teaching by making it synonymous with his own personal views," he wrote.

The bishop was disturbed that Mr. Kmiec's appearance was promoted by the Obama campaign, said Jerry Zufelt, director of communications for the Greensburg diocese.

In the Diocese of Scranton, Bishop Joseph Martino arrived unannounced at a parish forum that featured two McCain supporters, an anti-abortion Catholic who supported Mr. Obama, and a nun who didn't say who she supported but addressed many social issues.

Bishop Martino objected to copies of Faithful Citizenship being handed out at the forum. The document was passed 221-4 by the U.S. Conference of Catholic Bishops. "No USCCB document is relevant in this diocese," he told the gathering, saying that his own letter emphasizing abortion should have been distributed instead.

Two top officials of the USCCB, Cardinal Justin Rigali of Philadelphia, chairman of the Committee on Pro-life Activities, and Bishop William Murphy, chairman of the Committee on Domestic Justice, issued a response to arguments made by both Mr. Kmiec and Bishop Martino,

It criticized attempts to separate abortion laws from the need for social services. "Our faith requires us to oppose abortion on demand and to provide help to mothers facing challenging pregnancies," they wrote. Reversing Roe v. Wade "is not a mere political tactic but a moral imperative."

That said, "we wish to affirm that . . . Faithful Citizenship is the teaching that has been approved by the body of bishops of the United States."