Groups backing the overhaul this year have cobbled together a wide-ranging but fragile coalition that includes Latino and black groups, Roman Catholic and evangelical Christian churches, farm workers and commercial farmers, and some employer groups. In contrast to 2007, organized labor is united this time around in supporting the overhaul.

The political fault lines opened by Senator Leahy’s same-sex bill quickly became apparent this week. In a letter sent Tuesday, Bishop John C. Wester of Salt Lake City, the chairman of the Catholic bishops’ Committee on Migration, wrote that the Uniting American Families Act would “erode the institution of marriage and family,” by taking a position “that is contrary to the very nature of marriage which pre-dates the Church and the State.”

Bishop Wester addressed his letter to Representative Michael M. Honda, a California Democrat who has said he will introduce an immigration bill containing similar same-sex provisions in the House this week.

J. Kevin Appleby, the immigration policy director for the bishops’ conference, said, “The last thing the national immigration debate needs is another politically divisive issue added to the mix.”

But Senator Leahy said the bill would eliminate discrimination in immigration law against gay and lesbian couples.

Under family unification provisions in immigration law, American citizens and legal residents can petition for residency for their spouses. There is no numerical limit on permanent residence visas, known as green cards, for spouses of American citizens, and this is one of the main channels for legal immigration to the United States. Same-sex couples, though, cannot petition for partners, and many face the prospect of an immigrant partner’s deportation.

Senator Leahy’s bill would add the term “permanent partner” to sections of current immigration law that refer to married couples, and would provide a legal definition of those terms.