The national immigration debate is muddying Republican relations with Roman Catholics — coveted swing voters who constitute about one-quarter of the electorate.

While Catholic bishops and many Republican politicians share opposition to abortion, they're often split over the specifics of immigration reform. Church leaders are challenging — and in some cases even vowing to defy — the tougher enforcement proposals by GOP lawmakers.

The issue highlights the roadblocks that the Catholic worldview creates for Republicans and Democrats. Catholics generally are conservative on personal issues such as marriage, but they tend to be liberal on social-justice issues, limiting the appeal of both major parties and leaving Catholics "politically homeless," said the Rev. James L. Heft, president of the Institute of Advanced Catholic Studies at the University of Southern California.

"I'd like to see more pro-life Democrats," Heft said, "and social-justice Republicans."

Immigration is not the first issue to split the GOP and Catholic leaders.

Pope John Paul II opposed the U.S.-led war on Iraq and the death penalty, for example. But these latest differences have emerged only months before much of the Republican-controlled Congress is up for re-election, and when the GOP and Catholics had seemed closer than ever.

"Right now, a higher proportion of Catholic voters (than in the past) would identify with the Republican Party, or some of the themes that the Republican candidates have been using," said David Leege, a professor emeritus at the University of Notre Dame and an expert on Catholics and politics.

But the impact of the immigration debate is unclear. "The jury is out on the Catholic vote in the long run," Leege said.

Catholics, once solidly Democratic, have been moving toward the Republican Party for the past 25 years or so. When struggling Catholics established themselves financially, they started voting less according to religious ties and more according to economic interests. The Democrats' embrace of abortion rights also drew them to GOP candidates.

President Bush, a Methodist, won the 2004 Catholic vote 52 percent to 47 percent over Democratic nominee John F. Kerry, who is Catholic. Leading up to the election, bishops had warned Catholic lawmakers they risk "cooperating in evil" if they vote for candidates supporting abortion rights. Church leaders insisted their position was nonpartisan, yet their timing clearly was a boon to Republicans because Kerry backs abortion rights.

But now, many of these same bishops are accusing GOP lawmakers of lacking compassion for illegal migrants. St. Louis Archbishop Raymond Burke, who said in 2004 that he would refuse to give Holy Communion to Kerry, was among many church leaders who organized recent rallies in favor of giving illegal immigrants a chance at citizenship. Burke said American Catholics were immigrants themselves, and that by welcoming migrants, "we obey the command of Our Lord, who tells us that when we welcome the stranger, we welcome Christ himself."

Rep. James Sensenbrenner, a Republican from Wisconsin, galvanized Catholic opposition by sponsoring legislation that the House passed in December that would make it a felony to be in the country illegally, and making it a crime to help illegal immigrants.

Los Angeles Cardinal Roger Mahony said his priests would disobey such a law. Successive popes, including John Paul II, have stressed that nations with the resources to accommodate people fleeing persecution or economic hardship are morally obligated to do so — regardless of legal status. About 30 percent of the nearly 65 million U.S. Catholics are Hispanic, and the church has an extensive social service network for migrants.