By Karen Brooks

(Reuters) - President Obama is endorsing a proposal by the Illinois legislature to legalize gay marriage, a White House spokesman told Reuters on Sunday.

It's an unusual move by a president - most of whom rarely weigh in on state legislative matters. Obama served in the Illinois state senate.

Obama, who said earlier this year that he supports same-sex marriage, believes "it's wrong to prevent couples who are in loving, committed relationships and want to marry, from doing so," said White House spokesman Shin Inouye.

"Were the president still in the Illinois State Legislature, he would support this measure that would treat all Illinois couples equally," Inouye said.

Chicago state Senator Heather Steans will introduce a gay marriage proposal this week, said Rikeesha Phelon, spokeswoman for Illinois Senate President John Cullerton. She added that Democratic leaders are confident they have the votes to win approval - possibly even before the legislature's newly elected Democratic super-majority takes office on January 10.

If it passes, that would make Illinois the tenth state to approve same-sex nuptials.

Passage in President Barack Obama's home state would be a symbolic victory for gay rights activists, particularly after the president endorsed same-sex marriage in May.

One 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 legislative votes to legalize gay marriage there. Continued...