CHICAGO - Building on the momentum of President Obama’s recent support of gay marriage, two major civil rights groups filed lawsuits Wednesday in hopes of legalizing same-sex marriage in Illinois.

The two lawsuits - brought by the American Civil Liberties Union of Illinois and the New York-based Lambda Legal - represent 25 couples statewide. Both challenge a state law that defines marriage as between a man and woman, arguing the Illinois Constitution guarantees the right for same-sex couples to marry under due process and equality clauses.

“It’s time for Illinois to recognize the love and commitment of these couples,’’ said John Knight, director of the Lesbian, Gay, Bisexual, Transgender Project of the ACLU of Illinois.

Knight said Obama’s comments- as well as Governor Pat Quinn’s backing of gay marriage earlier this month and nationwide polls showing increasing public support for same-sex marriage - set the stage.

The filings come a year after Illinois enacted civil unions, but many couples in the lawsuits said civil unions made them feel like second-class citizens.

The 25 couples in both lawsuits tried to apply for marriage licenses in Cook County but were denied.

It is unclear how Illinois will handle the cases’ legal process, but attorneys with the advocacy groups are ready to take them to the state Supreme Court. The defendant named in the case, Cook County Clerk David Orr, is personally in favor of gay marriage.

The District of Columbia and six states - Connecticut, Iowa, Massachusetts, New Hampshire, New York, and Vermont - have legalized gay marriage. Courts decided for gay marriage in Massachusetts, Connecticut, and Iowa. A lawsuit, filed by Lambda Legal, challenging an Iowa law that barred gay marriage prompted the Iowa Supreme Court to legalize it in 2009.

Legislation to eliminate language banning gay marriage is pending in Illinois, but a vote is not expected before the session is scheduled to end this week.