When President Bush sits down with Pope John Paul II at the Vatican this morning, their meeting will highlight one of the most significant stories of the 2004 presidential campaign: the battle between Bush and Sen. John F. Kerry (D-Mass.) for the Catholic vote in America.

Rarely have politics and religion become so intertwined as they have this year in the pursuit of Roman Catholic voters. Still reeling from the scandals involving sexual molestation of children by Catholic priests, many parishioners are suddenly caught up in a clash between a handful of outspoken bishops and Catholic politicians who disagree with some of the church's central teachings.

Kerry will be the first Roman Catholic to win his party's presidential nomination since John F. Kennedy in 1960, but he is at odds with the church's position on abortion. Some bishops have said that Kerry and other politicians who share his views should be denied Communion. Rather than having to reassure non-Catholics that he will not be swayed by the Vatican, as Kennedy was compelled to do, Kerry is under fire from some Catholics for not being sufficiently under the sway of his faith.

Bush, a born-again Christian, started off his presidential campaign in 2000 having to make amends for a visit to South Carolina's Bob Jones University, which acknowledges being anti-Catholicism. But he has courted conservative Catholics and reached out to the church's hierarchy ever since to expand the Republican coalition. His appeals to evangelical Christians on social issues such as abortion and same-sex marriage are aimed at a segment of the Catholic population as well.

Since his election, Bush has systematically reached out to the Catholic community, with frequent meetings, conference calls and contacts at the White House. In addition, say some outside advisers to the White House, there is an unusual sensitivity among White House speechwriters and policymakers to Catholic positions on a range of issues.

"There's a very high level of awareness right up to the level of the president himself about what Catholic perspectives are," said Deal Hudson, publisher of Crisis magazine, who is close to White House senior adviser Karl Rove and other administration officials.

Today's meeting, however, will underscore the conflicting currents that come with pursuing Catholic voters. Bush has met previously with the pope, but this is their first meeting since the president began the war in Iraq, which the Vatican opposed. The White House requested the meeting and rearranged the president's schedule to make it possible, but it runs the risk that the pope or his top advisers could publicly repeat the Vatican's previous objections to U.S. "unilateralism."

The attention the White House has given to Catholic voters befits what has become one of the most prized swing votes in the country. Because of their geographic concentration, Catholics could determine the outcome of the election in such battleground states as Pennsylvania, Michigan, Missouri, Ohio, Wisconsin and Iowa. Recent polls show Catholics narrowly favoring Kerry over Bush.

Many analysts who have studied Roman Catholic voting patterns argue that there is no such thing as "the Catholic vote," since a group that constitutes nearly a quarter of the population is too large to move as a bloc. Catholic voters, particularly white Catholic voters, look and act much like the population at large.

Yet Catholics were once among the most loyal Democratic constituencies in the country. Catholic political solidarity was forged in the early decades of the 20th century in the face of anti-Catholic sentiment and culminated with the presidential bid of Alfred E. Smith, the first Catholic to secure the Democratic nomination, in 1928. When Kennedy ran for president in 1960, he won almost 80 percent of the Catholic vote, according to a Gallup poll at the time.

Since then, Catholics have become a consistent bellwether constituency, tipping toward the winning candidate in presidential elections through the 1970s, 1980s and 1990s -- until four years ago, when Al Gore narrowly carried the Catholic vote over Bush, according to exit polls, although Bush narrowly won the vote of white Catholics.

"The strong Democratic constituency that Catholics were in the 1940s and 1950s has . . . clearly disappeared," said John C. Green, director of the Ray C. Bliss Institute of Applied Politics at the University of Akron. "Catholics are evenly divided now."

Education and affluence changed the face of the Catholic Church in the United States in the second half of the 20th century and, with it, the voting patterns of its members. What once was a predominantly urban, ethnic constituency now is wealthier and more suburban.

Catholics remain more Democratic than Protestants. In presidential elections, Protestants vote in significantly greater numbers for the Republican candidate, and between 1980 and 2002, Catholics voted Democratic in congressional elections each time except the GOP landslide of 1994, according to exit polls.

But the Catholic vote is complex enough to defy easy categorization, and parishioners appear far more "cross-pressured" than other voters, said Democratic pollster Geoffrey Garin. Catholic voters seldom find candidates who reflect the church's conservative positions on such issues as gay marriage and its liberal teachings on social justice. Catholics are more likely to be part of an antiwar constituency than voters of other religions, but they are also more conservative on moral issues.

As a result, the Catholic vote is often broken down into Catholics who attend Mass weekly, a group Bush won decisively in 2000, and those who attend Mass less frequently, a group former vice president Al Gore won easily.

"The Protestant-Catholic split was once very real," said John Kenneth White, a professor of political science at Catholic University. "But the big split now is between those who go to church frequently and those who don't. Regardless of denomination, those in the pews are much more likely to vote Republican."

Both the Kerry and Bush campaigns have plans for outreach to religious voters. On Wednesday, watchdog groups publicized a Bush campaign e-mail seeking to enlist 1,600 "friendly congregations" in Pennsylvania where Bush supporters could "gather on a regular basis." IRS rules forbid tax-exempt religious organizations from endorsing candidates or engaging in partisan politics, but experts said the Bush effort will not run afoul of those rules if it is limited to voter registration and nonpartisan "issue education."

Matthew Dowd, a senior adviser to the Bush-Cheney reelection committee, said the very complexity of the Catholic vote makes him question whether there is any easy way for a politician to appeal to all Catholic voters.

Indeed, Bush and Kerry appear to be aiming their appeals at different parts of the 66 million-member Catholic community, with the president seeking support from conservative Catholics who attend Mass every week and Kerry counting on help from liberal Catholics and immigrants, particularly the country's 22 million Hispanic Catholics.

Several bishops have said politicians who stray from the church's teaching on abortion should be denied Communion, but there appears to be limited support among rank-and-file Catholics for such a penalty. A Washington Post-ABC News poll conducted May 20 to 23 found that 22 percent of Catholics supported the position of Catholic prelates such as St. Louis Archbishop Raymond Leo Burke, who said he would urge his priests not to give Communion to Kerry.

The poll found that 72 percent of all Catholics, and 58 percent of those who attend Mass weekly, opposed Burke's position. Evangelical Christians were far more supportive of Burke's position than Catholics were, according to the poll.

Ten days from now, the nation's bishops are expected to discuss the Communion issue and hear a progress report by a committee led by Cardinal Theodore E. McCarrick of Washington at a closed-door meeting in Denver. Based on public statements so far, only a small fraction of the 300 bishops favor denying Communion to politicians who support abortion rights, though many more think such politicians should voluntarily deny themselves Communion.

Catholic leaders want the church to have a strong voice in politics, but some worry that it could be dragged into a partisan battle at a time when American politics are growing increasingly acrimonious.

"We're in a time here in our country where there is more radical annoyance with anyone who doesn't take your side. There's almost a lack of civility . . . that creeps into every institution in society," McCarrick said. "Right now I think the church is okay. But the waters are swirling around us, and they're waters that are filled with distrust and with anger and with annoyance."

In the Post-ABC News poll, Bush received lower ratings from Catholics than from the general public on Iraq, the economy and his overall handling of the presidency. For example, 34 percent of all Catholics interviewed said they approved of Bush's handling of Iraq, compared with 40 percent of all adults. On the economy, 38 percent of Catholics said they trusted Bush more, while 56 percent said they trusted Kerry.

On their choice for president, Kerry led Bush, 53 percent to 41 percent, among all Catholics. But among white Catholics, the two were almost evenly split, a sign of how much the Catholic vote remains up for grabs.