SPRINGFIELD, Ill. — Illinois took a major step Tuesday night toward allowing civil unions for same-sex couples, a sign that gay rights keeps gaining momentum inside a Capitol where it languished for decades.

As onlookers broke out into cheers, the House for the first time ever approved civil unions, with one vote to spare. Supporters expect the Senate to follow suit Wednesday, and Democratic Gov. Pat Quinn campaigned this fall on a pledge to sign it into law.

“”We have a chance here, as leaders have had in previous generations, to correct injustice and to move us down the path toward liberty,”” said sponsoring Democratic state Rep. Greg Harris, one of two openly gay lawmakers, his voice breaking with emotion. “”It’s a matter of fairness, it’s a matter of respect, it’s a matter of equality.””

Opponents charged that civil unions are a “”slippery slope”” that will erode traditional family values.

“”Are you ready for gay marriage?”” asked Republican state Rep. David Reis, who raised his voice putting that question to colleagues.

Under the proposal, same-sex couples would enjoy several rights married couples currently have, such as making end-of-life decisions, handling probate matters, sharing nursing home rooms or even visiting partners in hospitals that deny visits by anyone but family.

Business groups did not weigh in on the measure. State officials say they expect some increase in health insurance costs.

“”Most of the major businesses in our country now extend domestic partnership benefits to their employees,”” said Rick Garcia, political director of Equality Illinois. “”The state of Illinois already extends domestic partnership benefits to employees. This really is cost-neutral.””

If approved, Illinois next summer would join New Jersey in having a civil union law on the books. Vermont, New Hampshire, Connecticut and Iowa have same-sex marriage laws, according to the National Conference of State Legislatures.

The civil unions success is the latest in a quickly evolving attitude about gay rights in Illinois. Only five years ago, lawmakers passed protections against discrimination in jobs and housing for gays and lesbians. It took decades to pass that measure. Illinois has moved toward more liberal stances on social issues since Democrats took control of state government at the start of 2003.

A Chicago Tribune poll conducted in late September showed 57 percent approved of legalizing civil unions while 32 percent disapproved.

Approval came despite vigorous opposition from the Catholic Conference of Illinois, which is headed by Cardinal Francis George, who personally made calls to legislators asking lawmakers to oppose the bill. But proponents waged a strong lobbying effort of their own.

Quinn, who is Catholic, took the extra step of standing on the House floor to watch the breakthrough vote. He called passage a “”great step forward.””

“”It’s important that we respect the diversity that we have in our state and be a tolerant state of Illinois,”” Quinn said.

During the debate, Democratic state Rep. Deborah Mell tearfully implored legislators to vote for the bill as her longtime partner, Christin Baker, sat near on the House floor. They plan to get married in Iowa next fall.

“”I love my state and am proud to live here. But my state does not treat me equally. It will take my money, take my taxes, I can even make laws for people,”” said Mell, the daughter of Chicago Alderman Richard Mell and sister-in-law of former Gov. Rod Blagojevich. “”If God forbid something happens to Christin, and she cannot make a decision, by law the doctor cannot ask me anything. I am not able to speak for her wishes. Under the law the doctor has to go to her family and I’m not considered family.””

Though unusually quiet during debate, the House chamber echoed at times with vocal dissent.

Republican state Rep. Ron Stephens said he wants to avoid having to “”someday explain to my children and grandchildren that no longer in America are we going to give the honor to a man and a woman in marriage.””

“”I believe that if this should ever pass, the next bill will be legalizing marriage between members of the same sex. And I just think that’s wrong. You might think I’m wrong in thinking that … just call me an old-fashioned traditionalist.””

Republican state Rep. Rosemary Mulligan offered a different view, saying she had been too sick to travel to Springfield for earlier legislative action, but she got in her car and drove to Springfield on Tuesday when she learned from supporters she might be the 60th vote needed to pass the measure.

“”The fact of the matter is there are gay people, and you’re not going to abolish the fact that they are gay by not letting them have these rights,”” said Mulligan, who added that she wanted to demonstrate that not all Republicans are strict conservatives when it comes to social issues. Still, the overwhelming number of the 61 lawmakers who voted for civil unions were Democrats.

Robert F. Gilligan, the Catholic Conference’s executive director, said he is disappointed that civil unions passed, but now is focused on lobbying senators to vote it down Wednesday. Senate President John Cullerton, a Democrat, is an avid supporter of the bill, and supporters have considered the House a tougher obstacle.

Gilligan said he was particularly dismayed that the bill passed during a lame-duck session. The measure got a boost from as many as a dozen lawmakers who will not return because they are retiring or were defeated in the Nov. 2 election.

Camilla Taylor, a lawyer with Lambda Legal in Chicago, a group that helped draft the legislation, said businesses in Illinois that provide spousal benefits will have to treat same-sex couples who have entered into a civil union the same as heterosexual couples.

“”The civil unions bill makes especially clear that businesses aren’t acting in good faith if they continue to treat same-sex couples differently with regard to spousal benefits,”” Taylor said.

She said that in other states where civil unions bills have passed, Lambda Legal has won cases in which businesses have refused to provide spousal benefits to partners in a civil union.

Modesto Valle, executive director of the Center on Halsted, a lesbian, gay, bisexual and transgender community center in Lakeview, Ill., said if civil unions become law, he expects people will take advantage of it in a measured fashion.

“”I don’t believe that all of a sudden people will be running out the door to be recognized in their union,”” he said. “”People are going to take this very seriously, just like marriage. Once marriage gets passed in this country, it’s not like all gay and lesbian people are going to run out and get married. It’s something to be taken very seriously.””