Seemingly out of nowhere, Illinois, the fifth largest state, with a population of almost thirteen million people, is likely about to become the tenth state to allow same-sex marriage. It could happen within the next week, during the lame-duck session of the state legislature, which ends on January 8th. If it does, over twenty per cent of the U.S. population will live in states with marriage equality, even before the U.S. Supreme Court rules on marriage rights in California this June. California alone has approximately twelve per cent of the U.S population, and its addition to the group would then bring the total to about a third of the population.

Illinois’ civil-unions law, in effect since June of 2011, is one of the most inclusive in the nation, and was considered a very significant political achievement eighteen months ago, when it went into effect. (It, too, passed during a lame-duck session.) But it has quickly been overtaken. Civil-rights advocates and elected officials who support same-sex marriage in Illinois are rapidly building on the momentum created by the first-ever voter-referendum wins in last November’s election—in Maine, Maryland, and Washington—and the unprecedented jump in support for equal marriage rights reflected in national and state polls. The legislature seems also to be moving in response to legal action by the A.C.L.U. and Lambda Legal challenging the civil-union statute. Both Illinois Governor Pat Quinn and Chicago Mayor Rahm Emanuel now support full marriage equality.

Quietly, right after Christmas, President Obama, who before serving in the Senate had been in the Illinois legislature, endorsed the marriage proposal now working its way through that body, with a spokesman saying, “Were the President still in the Illinois State Legislature, he would support this measure that would treat all Illinois couples equally.” It was the first time he had endorsed a state-legislative effort for marriage equality, and may have been the result of requests from major Illinois-based Democratic donors, other supporters, and long-time friends.

Right now, advocates plan to hold a State Senate hearing on the legislation as soon as later today, January 2nd, and the bill could be voted on by the full chamber on Thursday, January 3rd. The State House of Representatives would then have through Wednesday, January 8th, to pass the bill during the lame-duck session. Advocates are confident that they have the votes in the State Senate and feel that they are “within striking distance” in the House, according to a source close to the situation. Governor Quinn has said that he would sign the law. Although advocates want to get a vote now, during the lame-duck session, because they feel a win is likely, the new legislature being sworn in on January 9th has a supermajority of Democratic members who would also presumably favor of the legislation.

Illinois, being the President’s home state and containing four per cent of the total U.S. population, would be a big win for marriage-equality advocates. If it does come early in January, as is now expected, it would be much sooner than anticipated. Marriage efforts in Delaware, Hawaii, Minnesota, New Jersey, and Rhode Island are also getting attention. Iowa, the other Midwestern state that allows gay marriage, does so as the result of a state-supreme-court decision in 2009, not a ballot initiative or legislative action. Its passing would send one more signal to the Supreme Court as it turns its attention to the issue of same-sex marriage.

Over the weekend, Chicago’s Catholic Cardinal Francis George sent a strongly worded letter to followers, urging them to contact their state legislators to oppose the marriage bill. “Civil laws that establish ‘same sex marriage’ create a legal fiction,” he wrote. “The State has no power to create something that nature itself tells us is impossible.” The fact that the Catholic Church has stepped up its opposition is seen as a sign that events are moving very quickly.

Illustration by Tom Bachtell.