— The nation’s Roman Catholic bishops are in a difficult position as the debate over immigration reform gets underway: The immigrant-built American church, known for advocating a broad welcome for migrants and refugees, could end up opposing reform because it would recognize same-sex partners.

Proposals by President Barack Obama and the Congressional Hispanic Caucus include the same-sex partners of Americans among those who would be eligible for visas. The Human Rights Campaign and other gay advocates welcomed the recognition, arguing current laws unfairly treat people in gay or lesbian relationships “as strangers.” The idea has the backing of the National Council de la Raza and other liberal Latino groups.

But Catholic bishops, with the support of evangelicals and other theological conservatives, have sent a letter to Obama protesting his proposal. In a sign of the sensitivity of the issue, the U.S. Conference of Catholic Bishops would not provide a copy of the statement, saying the signatories agreed not to make the letter public. Sister Mary Ann Walsh, a spokeswoman for the bishops, would say only that recognition of gay couples in the president’s reform proposals “jeopardizes passage of the bill.”

Galen Carey, public policy officer for the National Association of Evangelicals, which represents 40 denominations and has been lobbying for new immigration laws, said, “Our view is immigration reform is not the place to have this discussion.” The theologically conservative Lutheran Church-Missouri Synod also signed the Catholic bishops’ letter.

“The issue of immigration on its own is so controversial, so polarizing,” said the Rev. Samuel Rodriguez of the evangelical National Hispanic Christian Leadership Conference. He was in the Las Vegas audience last week when Obama presented his plan. “Let’s not play politics with 11 million undocumented immigrants.”

It is far too early to know how much of a factor gay relationships will become in what is expected to be a complicated and emotional debate. The plan unveiled last week by eight leading Democratic and Republican senators did not mention same-gender partners. Many other major religious groups lobbying for reform, such as The Episcopal Church, either support gay marriage or don’t make homosexuality a focus. In a conference call this week with reporters, White House Domestic Policy Council director Cecilia Munoz was asked whether Obama would support a bill that didn’t acknowledge same-sex partners. Her only response: “The president’s position on that is very clear.”

Still, endorsements from traditional denominations may carry more significance in the current political climate, in which conservative-leaning lawmakers are worried about political damage from backing a path to citizenship for illegal immigrants.

“The bishops’ support, I think, is going to be critical for swinging moderates in the House to support this bill,” said Stephen Schneck, a political scientist at Catholic University of America and chair of the anti-abortion Democrats for Life, who was part of the Catholics for Obama re-election effort.

It seems unlikely the bishops would accept any provision for same-sex partners— even for an issue as important to the church as immigration. In their drive for greater orthodoxy among Catholics, bishops have made preserving traditional marriage a priority. Last week, San Francisco Archbishop Salvatore Cordileone, who leads the bishops’ marriage efforts, said the struggle against gay marriage is a gift from God “and by overcoming it we may achieve spiritual greatness.” He made the comments in an interview with The Catholic Herald, a news outlet in Britain.

The bishops’ stand against Obama’s health care law provides some indication of their thinking when they view a core moral teaching in conflict with a long-held social justice goal.

For decades, the bishops had advocated for improved access to health care, especially for the poor. But church leaders concluded that the president’s plan, known as the Affordable Care Act, would provide financing for ending pregnancies. The administration and Democratic supporters of the law insisted the bishops were wrong, and said no taxpayer money would fund abortion coverage. But the bishops ultimately opposed the legislation.

Yet, immigration seems even more critical than health care to the church.

Americans church leaders have spent decades lobbying for revisions that would keep families together and fulfill what the church considers the duty of all countries, especially wealthier ones, to do as much as possible to help the poor and persecuted. The church and Catholic groups run a network of aid programs for migrants, refugees and illegal immigrants, taking positions that recognize the country’s right to protect its borders, but that still fall “to the left of the Democratic Party,” Schneck said.

This position is rooted in papal and Gospel teachings so extensive that evangelicals often borrow the theological framework for their own advocacy. In a 2003 joint plea for immigration reform, called “Strangers No Longer,” U.S. and Mexican bishops stated, “Regardless of their legal status, migrants, like all persons, possess inherent human dignity that should be respected.”

The issue is of special historic importance to the American Catholic church, which was built by waves of Irish, Italians, Poles and others. The immigrant presence in the pews is now growing as American-born white Catholics drop out in significant numbers. Researchers estimate that a third of the 66 million U.S. Catholics are Latino.

“This is an issue that has been a huge priority for the church for a really long time,” said Kristin Heyer, a professor at Santa Clara University in California who studies immigration and Catholic social thought. “The wider Catholic community, in addition to the bishops, has mobilized in a major way.”

Ultimately, the controversy could split Catholics, in much the same way that Catholics divided over health care. Despite enormous pressure from the bishops, the Catholic Health Association, a trade group that represents hospitals, provided critical backing for the president’s health care legislation. Surveys have found that large majorities of lay Catholics back same-sex marriage or civil unions.

Given the importance of Latinos to the U.S. church, political observers wonder how bishops could explain their opposition to Hispanic parishioners.

Kim Daniels, an attorney and director of Catholic Voices USA, a conservative-leaning lay group that defends church teaching, has been urging Catholics across the political spectrum to drop their differences and get behind immigration reform. Still, she said, “being Catholic in the public square means standing up for all our issues.”