Catholic bishops' leader backs constitutional ban on same-sex marriage

Introduced by Senator Wayne Allard, Republican of Colorado, the measure defines marriage as "the union of a man and a woman" and is expected to come before the Senate around July 12.

Gregory asked bishops to urge their senators to get behind the amendment, and to encourage priests and parishioners to do the same.

"This situation challenges Catholics -- and all who seek the truth -- to think deeply about the meaning of marriage, its purposes and its value to individuals, families, and society," Gregory wrote.

Catholics are the largest denomination in the country, with 63.7 million members.

The Southern Baptist Convention, the nation's second-largest denomination with 16.3 million members, has also backed an amendment against gay marriage, as have several other conservative religious groups.

Conservative Protestants are organizing "Marriage Protection Sunday" July 11, encouraging pastors and religious educators to address the topic, then mobilize congregants to lobby their senators the next day.

Several other denominations oppose a constitutional amendment.

Twenty-six religious organizations, including the Episcopal Church, Evangelical Lutheran Church in America, and the Union for Reform Judaism, have together urged Congress to reject the legislation.

Although several Senate opponents of the amendment are Catholic, Gregory, of Belleville, Ill., said nothing in his letter about any obligation they might have to vote for the measure.

The Vatican last year said Catholic politicians have a duty to uphold the church's "nonnegotiable ethical principles" -- specifically mentioning opposition to abortion and recognition for same-sex couples.

Amendments to the Constitution require approval by two-thirds majorities in the House and Senate and ratification by three-fourths of the state legislatures.