UPDATED 11/30/10 – 5:36 p.m.

SPRINGFIELD, Ill. (CBS) – The Illinois House has begun debating a proposal to legalize civil unions for same-sex couples.

Supporters and opponents were working throughout the day Tuesday to line up votes for and against the measure to offer legal recognition to same-sex couples.

The House vote is expected to be extremely close and, if the House approves the measure, a Senate vote is likely to follow later this week. A Senate committee approved a separate civil union measure earlier Tuesday.

Backers say civil unions are a simple matter of fairness. They would grant gay couples some of the same legal rights that come with marriage.

Opponents argue civil unions are just same-sex marriage by a different name. They say Illinois shouldn’t take any step in that direction.

The House begun debate on Senate Bill 1716 late Tuesday afternoon. The bill is co-sponsored by openly gay state Rep. Greg Harris (D-Chicago.)

Civil unions would provide legal recognition of gay couples and give them some of the same benefits automatically available to married couples – the right to visit a sick partner in the hospital and make decisions about their medical care, for instance. Backers consider it a matter of fundamental fairness.

Supporters say they’re close to having the 60 votes needed to pass the measure in the House. If it passes there, backers think the Senate quickly would follow suit and Democratic Gov. Pat Quinn would sign it into law.

“I do think this is the time for Illinois to do this,” Quinn said Monday. “This is a good way to show employers – big businesses all across the country – that Illinois treats everyone with tolerance.”

On Monday evening, the gay rights group Equality Illinois put out a bulletin asking for help in phone banking at its offices at 3318 N. Halsted St. Rick Garcia, the public policy director for the group, told ChicagoPride.com that supporters are “just a few votes away in the House for victory.”

Jacob Meister, the board chairman at The Civil Rights Agenda and a former U.S. Senate candidate, told ChicagoPride.com he is “feeling very good” about the odds the bill will pass. The group contacted more than 16,000 GLBT voters and allies, and Meister said they have made inroads in “swing and persuadable districts,” ChicagoPride.com reported.

Mayoral candidate Gery Chico also got involved, contacting state legislators to support the bill. Candidates Rahm Emanuel and Carol Moseley Braun have also spoken in favor of civil unions.

But critics argue that civil unions would amount to gay marriage under another name, and they are fighting hard to defeat the bill.

“They have a full-court press on, and we have a full-court press, too,” said David E. Smith, executive director of the Illinois Family Institute.

Smith said the Family Institute has made hundreds of thousands of automated calls to voters in recent months, urging them to contact their legislators. The group has sent bulletins on the subject to hundreds of Illinois churches, and Smith said he is lobbying lawmakers directly. Along with other conservative organizations, the institute is sending e-mails to everyone on its lists to call for action against civil unions.

Meanwhile, the Catholic Conference of Illinois is fighting the legislation, too. It’s urging Catholic churches across Illinois to speak out, said Executive Director Robert Gilligan, and bishops are contacting lawmakers to speak against the measure.

“Marriage is what it is and always has been, no matter what a legislature decides to do,” Francis Cardinal George said in a statement last week.

Even though the civil unions bill would not legalize same-sex marriage, George said in a news release that “the public understanding of marriage will be negatively affected by passage of a bill that ignores the natural fact that sexual complementarity is at the core of marriage.”

The National Organization for Marriage has also been campaigning against the bill.

Opponents make several arguments against civil unions.

Some say it’s simply wrong. “It’s not legitimate for the government to recognize the relationship of homosexual partners or polygamous partners or incestuous partners,” Smith said.

They also warn the legislation could hurt religious institutions, even though its official name is the “Illinois Religious Freedom Protection and Civil Union Act.” The measure wouldn’t require churches to recognize civil unions or perform any kind of ceremony, opponents acknowledge, but they fear it would lead to other requirements, such as including same-sex couples in adoption programs run by religious groups or granting benefits to employees’ partners.

Opponents also see the measure as an attempt to change the definition of marriage. They note it says civil partners will have the same rights and responsibilities that Illinois grants to spouses, and that civil partners will be included in the legal definition of “spouse.”

But Garcia rejects the argument that civil unions are simply a form of gay marriage. He’s also trying to make sure the public knows that religious leaders aren’t uniformly against the measure. He says 25 ministers, nuns and rabbis who support civil unions will appear at a news conference Tuesday and will discuss a letter of support signed by some 300 other religious leaders.

In a notice on its website, the leaders at Equality Illinois said they are also “deeply disturbed” at claims by George that the bill will infringe on “religious liberty” by forcing religious organizations to recognize civil unions.

“The bill explicitly states that: ‘Nothing in this Act shall interfere with or regulate the religious practice of any religious body. Any religious body, Indian Nation or Tribe or Native Group is free to choose whether or not to solemnize or officiate a civil union,’” the notice said.

George said in his comments that the bill could force faith-based institutions to place adoptive children with same-sex couples, force Catholic parishes or social service agencies to provide services to couples in civil unions, or refuse to protect small employers that will not extend family benefits to employees with same-sex partners.

Garcia and Harris labeled all the opponents’ arguments scare tactics.

“I think it’s easier to instill fear in people than it is to instill hope,” Harris said.

They said even if civil unions are approved, Illinois law still would define marriage as the union of one man and one woman. Illinois civil unions would not be recognized in some states or by the federal government for tax purposes. Noting five states and the District of Columbia already offer civil unions, they dismissed fears it would interfere with religious institutions or affect marriage in any way.

“This is the right time to pass the civil unions bill,” Equality Illinois said. “Every single day, loving, committed same-sex couples across the entire State of Illinois are being denied nearly 650 protections that are otherwise available to married heterosexual couples….These are not protections that we take for granted.”

(TM and © Copyright 2010 CBS Radio Inc. and its relevant subsidiaries. CBS Radio and EYE Logo TM and Copyright 2010 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed. The Associated Press contributed to this report.)

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