The Maryland House of Delegates is expected to begin debate on a gay marriage bill on Wednesday.

Backers will fight to keep the bill, approved two weeks ago by the Senate, free from amendments, allowing the House to send the legislation directly to Governor Martin O'Malley, who has promised to sign it into law. Any changes would send the bill back to the Senate for its OK.

House Speaker Michael Bush has warned that any changes to the bill could kill it.

Among the proposed amendments expected are some that would broaden the bill's religious exemptions and an amendment that would recognize gay and lesbian couples with civil unions, instead of marriage.

The measure stumbled out of the House Judiciary Committee last week after two members altered their votes and a third wavered.

Supporters concede they remain a few votes shy of a win in the Democrat-controlled House.

Over the weekend, the House's six openly gay delegates urged colleagues to approve the measure.

“Please vote yes,” the members wrote in a letter. “Vote yes because you know it is the right thing to do. Vote yes because you want to stand on the right side of history. Vote yes because every family in Maryland needs the protections that marriage provides.”

Opponents of gay marriage are widely expected to mount a referendum on the law, if approved by lawmakers.