Illinois lawmakers began considering a measure on Wednesday that would make President Barack Obama's home state the 10th in the nation to legalize gay marriage.
Supporters and opponents furiously lobbied lawmakers as a leading sponsor of the proposal pressed for a quick vote in the state Senate. The "lame duck" session is the final meeting before a newly elected legislature takes office later in January.
Buoyed by November election referendum victories in Maryland, Maine and Washington state, supporters of gay marriage want to make Illinois the first Midwestern legislature to approve it. Iowa's Supreme Court legalized it in 2009.
If approved, Illinois would be the second most populous state to allow gay marriage after New York.
Democrats hold a majority in both chambers of the Illinois legislature. But as in Maryland, Washington state and New York, a few Republican votes may be needed to pass a bill in Illinois.
State Republican party chairman Pat Brady was making calls to Republican lawmakers in support of gay marriage, legislative sources said, which could help win some votes for the measure.
Obama, a former Illinois state senator, publicly endorsed gay marriage in Illinois over the weekend, a rare occasion when he has weighed in on a state matter.
On the other side of the issue, Chicago Cardinal Francis George sent a letter to Catholic parishes saying same sex marriage undermined the "natural family" between a man and a woman.
"The state has no power to create something that nature itself tells us is impossible," he wrote. The letter, signed by George and six auxiliary bishops, urges Catholics to reach out to their state legislators.
Last week, Senate President John Cullerton's said through a spokeswoman that he was confident of the votes to pass gay marriage.
CIVIL UNIONS ALREADY LEGAL
But a move on Wednesday to speed consideration of the proposal in the Senate narrowly failed, 28 to 24.
It was not clear if the procedural vote was an indication that the proposal was short of the votes needed to pass or if some lawmakers simply wanted to take more time for debate. The Illinois House will convene later in the week.
Even if Illinois lawmakers fail to approve gay marriage before a new legislature takes office, there is a reasonable chance of passage later in the year because Democrats gained seats in the November election and will have super-majorities in both chambers.
In June, 2011, Illinois legalized civil unions, which grant some of the rights of marriage to same-sex partners. But gay rights activists said that did not go far enough.
All prominent Democrats in Illinois have endorsed gay marriage, including Chicago Mayor Rahm Emanuel and Governor Pat Quinn.
A key issue to be resolved is whether Illinois should allow religious groups the option of declining to perform same-sex marriages. New York granted such an exception in 2011 in order to secure the votes to legalize gay marriage there.
A bill introduced in the Illinois House offers such a religious exemption.
Last week, at least 260 Illinois Jewish and Protestant leaders published a letter supporting same-sex marriage.
"There can be no justification for the law treating people differently on the basis of sexual orientation or gender identity," the letter said.
A survey of Illinois voters by Democratic firm Public Policy Polling late last year found 47 percent would allow gay marriage, 42 percent opposed and 11 percent not sure.
The poll of 500 Illinois voters from Nov. 26 to 28 had a margin of error of 4.4 percent.
In addition to the three states which voted in November to legalize gay marriage, six others allow it - Iowa, Massachusetts, Vermont, New York, Connecticut and New Hampshire, plus the District of Columbia.
Additional reporting by Mary Wisniewski; Writing by Greg McCune; Editing by Todd Eastham