WASHINGTON – Cheers rang out in the marble hallway of the Maryland State House as Gov. Martin O’Malley signed a gay marriage law and handed the pens to gay members of the General Assembly who were gathered behind him.

“For a free and diverse people, for a people of many faiths, for a people committed to the principle of religious freedom, the way forward is always found for the greater respect of the equal rights of all, for the human dignity of all,” O’Malley said before sitting down to sign the law.

The bill narrowly passed the House of Delegates on Feb. 17 and the state Senate less than a week later, and it now faces a campaign by its opponents to put it to a public vote in November. If that challenge fails, the law will be enacted on Jan. 1 next year.

After a similar measure died in the state House of Delegates last year, O’Malley, a Democrat, announced in July his intention to sponsor a new law. This year’s version of the bill included protections for churches and other religious institutions, allowing them to opt out of holding gay weddings.

In debate on the bill, opponents questioned whether the protections were strong enough, and a last-minute attempt to broaden the exemption was defeated in the Senate.

The Maryland Marriage Alliance, a coalition of religious groups, is leading the campaign to overturn the law at the polls. The organization is gathering petition signatures, the first step in the referendum process. The organization is linked to the powerful National Organization for Marriage, a group formed to fight for California’s Proposition 8 ban on same-sex unions.

Marylanders for Marriage Equality, a coalition that backs same-sex marriage rights, expects the efforts to get the referendum on the ballot to succeed and is mounting a campaign to uphold the law.