Messages left for a half-dozen legislators Thursday were not immediately returned.
Last month, prominent pastors of several black Chicago churches launched their opposition with 60-second commercials on black radio stations. The group, called the African American Clergy Coalition, includes former state senator the Rev. James Meeks.
The group was scheduled to meet Friday with Cardinal Francis George who also opposes same-sex marriage on moral grounds. George is the head of the Archdiocese of Chicago, which serves more than 2 million Roman Catholics.
In one ad, Bishop Larry Trotter of the Chicago's Sweet Holy Spirit — which boasts up to 9,000 members —tells listeners to urge lawmakers to vote no.
"I, too, am opposed to same sex marriage as you and every Christian should be," he says. "Marriage was the first institution created by our God. He tells us in the word that marriage should be between a man and a woman and not those of the same sex."
Pastors at Thursday's event countered religious arguments by playing up legal and civil rights.
"This is about equality and justice. This is a matter of equal protection under the law for all citizens. This is not a religious issue." said the Rev. Richard Tolliver of St. Edmund's Episcopal Church in Chicago. "Nothing in your church will change."
The legislation says that religious institutions can't be forced into performing ceremonies.
Advocates — including the American Civil Liberties Union and Lambda Legal — have pushed a diverse and intense campaign for months, calling on Hollywood celebrities, businesses, congressional leaders and lawmakers. They say opinions on the matter are rapidly shifting and point to U.S. Sen. Mark Kirk, the second sitting Republican senator to recently step forward in support of gay marriage.
"We're really close, certainly within striking distance," said Bernard Cherkasov the head of Equality Illinois, who noted the group was focusing efforts on black and Latino caucuses. "This is going to be a strong bipartisan vote. "
He said Kirk's support would resonate with moderate Republicans.
Gov. Pat Quinn, a Chicago Democrat, has said that he would sign the legislation if it comes to his desk. It would make Illinois the 10th state in the nation to allow same-sex marriage. The state approved civil unions in 2011.