By Joanne von Alroth

CHICAGO (Reuters) - 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. Continued...