“We dedicate our lives to fostering faith and compassion, and we work daily to promote justice and fairness for all,” the leaders wrote in the letter, which was released Sunday. “Standing on these beliefs, we think that it is morally just to grant equal opportunities and responsibilities to loving, committed same-sex couples. “There can be no justification,” they continued, “for the law treating people differently on the basis of sexual orientation or gender identity.” This is not the first time members of the clergy have endorsed same-sex marriage, but the public nature of the letter and the number of signatures made it an especially strong statement. The timing is also significant: State Senator Heather A. Steans and State Representative Greg Harris, both Democrats, plan to introduce a bill next month to legalize same-sex marriage. Ms. Steans said they would not put the legislation, the Religious Freedom and Marriage Fairness Act, up for a vote unless they were confident it would pass. She added that the Senate, at least, was “definitely within striking distance” of the 30 votes needed for passage and that she hoped the letter would help persuade undecided legislators to support the bill. Many of the 260 Christian and Jewish leaders who signed the letter said they had long supported same-sex marriage and were excited to make their views more public. “It’s not a religious right — it’s a civil right,” said the Rev. Kevin E. Tindell, a United Church of Christ minister at New Dimensions Chicago. “It’s a matter of justice, and so as a Christian, as a citizen, I feel that it’s my duty.” Mr. Tindell, who is gay, is raising three children with his partner of 17 years. The Rev. Kim L. Beckmann of the Evangelical Lutheran Church in America, who lives in the Chicago area, said she was drawn into the movement “as my gay and lesbian parishioners were welcomed into our congregation.” “I have participated in blessings of these unions for longer than we’ve even been talking about marriage,” she said. “I’m thrilled to take this step.” Laurie Higgins, cultural analyst for the Illinois Family Institute, which opposes same-sex marriage, criticized the branding of the issue as a matter of “equality” and “inclusion.” “All adults, regardless of their sexual proclivities, are entitled to participate in the sexually complementary institution of marriage,” she wrote in an e-mail. “Those who identify as homosexual choose not to participate in it.” The letter, Ms. Higgins said, “is signed quite obviously by faith leaders who have adopted radical, ahistorical, heretical theological views.” “Their views are informed not by careful exegesis, but by personal desire and political convictions,” she said. Signatories of the letter said one of their motivations was to challenge the assumption that religion went hand in hand with opposition to same-sex marriage. The Rev. Kara Wagner Sherer of St. John’s Episcopal Church in Chicago said it was a way for religious leaders to say, “I’m a faithful Christian or a Jew or Muslim, and I think that marriage equality is important.” “It doesn’t have to be a faith issue,” she said. “We understand our Scripture in a different way.” The Episcopal Church endorsed same-sex marriage in July. Other denominations, including the Presbyterian Church (U.S.A.) and the Roman Catholic Church, have reaffirmed their opposition. Ms. Steans said she and Mr. Harris had been careful to ensure that the Illinois legislation would protect religious freedom. Under the proposed law, she said, “no faith has to solemnize a marriage they don’t want to.” She added, though, that she had long believed that many religious leaders would like to conduct same-sex marriages, and that with the release of the letter, it was “very heartening to see that that will be the case.” Ms. Beckmann, the Lutheran minister, also cited the leeway for denominations and congregations to choose whether to ordain same-sex marriages.