By Joanne von Alroth

SPRINGFIELD, Illinois (Reuters) - The Illinois state Senate approved a bill to legalize same-sex marriage on Thursday in a Valentine's Day vote spearheaded by Democrats, as gay couples around the country used the romantic day to dramatize their quest for the right to marry.

The Illinois Senate, which is heavily Democratic, voted 34-21 to advance the measure to the House in President Barack Obama's home state. The fate of the bill in the state's lower chamber remains uncertain.

Democratic Governor Pat Quinn has promised to sign any bill legalizing gay marriage, which would make Illinois the 10th state to legalize same-sex nuptials, in addition to the District of Columbia. It would also become the first Midwestern state to approve same-sex marriage through legislation. Iowa's Supreme Court legalized such marriages there in 2009.

Obama has encouraged the drive to legalize gay marriage, authorizing a White House statement recently saying that if he were still in the Illinois legislature, he would vote for it.

The Illinois drive to legalize gay marriage coincided with a national campaign by a coalition of gay rights groups to highlight the issue on Valentine's Day.

As part of what they call "Freedom to Marry Week," same-sex couples will request marriage licenses in 18 places around the country, including a number of states where same sex-marriage is not legal.

"These laws are unjust and immoral, and we are confronting those laws head-on across the country," said Heather Cronk, managing director of GetEQUAL, a gay rights group.

Although the Illinois House, like the state's Senate, is controlled by Democrats, the fate of the legislation in the lower chamber remains a question mark.

House Republican Leader Tom Cross said he was not sure whether it would gain Republican support. There also was concern that some black Democratic legislators from Chicago could oppose the measure because of pressure from African-American Christian pastors.

Some black ministers have joined Illinois Catholic bishops in expressing strong opposition to legalization.

And within the Democratic party, gay marriage is still a divisive topic.

Two Democratic state lawmakers broke with their party leaders on Thursday and introduced measures in both chambers of the legislature that call for an amendment to the state constitution to enshrine marriage as between a man and a woman.

(Writing by Greg McCune; editing by Matthew Lewis, Cynthia Johnston and Leslie Adler)