WASHINGTON (By Adam Nagourney, NYTimes) April 3, 2005 - The death of Pope John Paul II came at a time when leaders of the Roman Catholic Church, reflecting the tone set by the Vatican, have become increasingly assertive in American political life. Their stance has created strains with some Catholic Democrats just as the White House has sought to make inroads with the once solidly Democratic Catholic constituency.

Several Catholic academics and elected officials said on Sunday that the shift - highlighted last year when some church leaders said Senator John Kerry should be denied communion because he supports abortion rights - reflected the tone set by a pope who was known for being conservative and for being willing to confront governments to press his views. They said the choice of the next pontiff could thus prove nearly as important for American political life as for the Vatican itself, as Democrats and Republicans here face increasingly pitched battles over judicial nominations, abortion, gay rights and euthanasia.

One of the potential successors to John Paul is Cardinal Francis Arinze of Nigeria, who during last year's presidential campaign said a politician who supported abortion "is not fit" to receive communion.

"I've seen an increase in directly political kinds of activity: what the responsibilities of Catholics are, how they should vote, etc.," said Mark J. Rozell, a professor of public policy at George Mason University and the author of numerous books and articles on religion and politics. "Some Vatican officials are pressing for more direct activism in public life and are willing to be more critical of public policy figures who do not take what they consider to be the right positions on policy issues."

John T. McGreevy, a professor of history at the University of Notre Dame, said: "John Paul had this culture of life vision. And that has sort of radicalized and emboldened some bishops."

The attempt by some Catholic Church leaders to influence American policy goes back at least to the 1930's, when bishops pressed President Franklin D. Roosevelt to create Social Security. The notion of church activism was fostered by the reforms of the Second Vatican Council 40 years ago, which encouraged bishops to advocate government programs to help the poor.

But in recent years, the emphasis has shifted away from fighting poverty and standing up for civil rights, issues associated with the Democratic Party, and toward issues like opposition to abortion, gay rights and euthanasia, issues that Mr. Bush and the Republicans have embraced.

At times last year, this assertiveness went beyond policy and into electoral politics, as some bishops hinted that their parishioners should vote for Mr. Bush instead of Mr. Kerry. At St. Patrick's Church in Wareham, Mass., for example, parishioners said that at the Saturday evening Mass right after the death of the pope, they were given pamphlets notifying them that they would be asked next week to sign postcards to Mr. Kerry and the state's other Democratic senator, Edward M. Kennedy, reading: "Please do not make support of the U.S. Supreme Court's abortion decision a litmus test for judicial nominees."

The rising assertiveness of some church leaders is particularly significant for American politics because President Bush has been making a concerted effort to win support among Catholic voters. Mr. Bush's efforts are part of an overall drive by his chief adviser, Karl Rove, to make inroads among typically Democratic groups of voters.

Mr. Bush assembled a group of Catholic advisers and began meeting with them regularly as soon as he entered the White House. Shortly after the Vatican announced the death of the pontiff, Mr. Bush and Laura Bush walked in unannounced to a Saturday afternoon Mass at the Cathedral of St. Matthew the Apostle. Mr. Bush's aides said Sunday that he expected to attend the funeral but had not made formal plans pending an invitation from the Vatican.

In talking about the dispute over the fate of Terri Schiavo, the severely brain-damaged Florida woman who died last week, Mr. Bush borrowed what had been a signature phrase of John Paul, as he talked about guarding "the culture of life," and he has forcefully embraced the Vatican's views on abortion, stem cell research and gay marriage.

"It is remarkable that a Republican non-Catholic president would be echoing this papal phrase," Professor McGreevy said. "Clearly, focusing on abortion and end-of-life issues, that was a big issue of this pope."

There is evidence that Mr. Bush has had some success in this regard. In last year's presidential election, Catholics supported Mr. Bush over Mr. Kerry, a Roman Catholic, by 52 percent to 47 percent, according to a survey of voters leaving the polls. Catholics made up 27 percent of the electorate last year, but they are disproportionately clustered in swing states, particularly Ohio and Pennsylvania, making them an important target in presidential campaigns.

Roman Catholics, however, are hardly a unified bloc of voters, and some analysts suggested on Sunday that the prominent bishops at the forefront of these recent battles are more conservative than most Catholic voters or, indeed, their fellow bishops.

Most polls show that Catholics support at least some access to abortion. And Mr. Bush's success with those voters last year came among the more devout: regular church-going Catholics supported him by 56 percent to 43 percent. Catholics who attend church less than once a week supported Mr. Bush by 50 percent to 49 percent. Mr. Kerry was the third Catholic who has won a major party's nomination for president; Kennedy was the only Catholic to have been elected president. After Mr. Kerry's defeat, a number of Democrats said they could not see how in this environment a Catholic Democrat could win the presidency, given that he or she would almost certainly have to support abortion rights to win the nomination.

There is arguably no other religion that has had such a contentious relationship with American politics, and Mr. Kerry is certainly not the first roman Catholic politician who has had difficulty with his church because of his views on issues of abortion. Most famously, former Gov. Mario M. Cuomo of New York tangled with Cardinal John J. O'Connor of New York after the cardinal said he did not see how "a Catholic in good conscience can vote for a candidate who explicitly supports abortion."

Last May, 48 Democratic Catholics in Congress sent a letter to Cardinal Theodore E. McCarrick of Washington, the chairman of the Task Force on Catholic Bishops and Catholic Politicians, expressing concern about the threat of withholding communion from Catholic politicians who support abortion rights. One of the signers, Representative Rosa DeLauro, Democrat of Connecticut, said she hoped the change of leadership at the Vatican would result in the church's turning its attention to what had been a traditional concern: "the great traditions of economic and social justice."

Some Catholic leaders suggested it would be a mistake for the next pope to continue down the road of involvement in politics and policy.

"I really think it was counterproductive to have done what was done in the last election," said Margaret O'Brien Steinfels, a former editor of Commonweal, the magazine of the Catholic laity. "I don't think it's going to work. I think it's a mistake for bishops to get involved to that degree in policy. I don't think the bishops should tell people who they should vote for. All they do is dilute their own credibility."