The National Organization for Marriage (NOM) is calling on the Maryland Senate to reject a gay marriage bill approved Friday by the House of Delegates.

After nearly 3 hours of debate, House members approved Governor Martin O'Malley's plan to make Maryland the eighth state to legalize gay marriage with a narrow 72 to 67 vote.

Supporters faced their greatest obstacle in the House, where Democratic leaders last year shelved a vote due to lack of support.

The Senate, whose composition has not changed, last year approved the bill.

NOM, the nation's most vociferous opponent of gay marriage, vowed retribution against lawmakers who vote for the legislation.

“The people of Maryland do not support same-sex marriage,” said NOM President Brian Brown in a statement. “We call on the Maryland Senate to reject this legislation. Failing that, we are committed to work with allies to ensure that the people of Maryland will decide this issue just as voters in 31 other states have done, which is to preserve marriage as the union of one man and one woman.”

“Legislators who vote to redefine marriage will be held accountable to their constituents. Those Republicans in the Assembly who abandoned principle and traded their votes will be held to account. Upwards of 80% of Republican voters oppose same-sex marriage. Democrats, too, will have to answer to their constituents, including the upwards of 70% of African-Americans who oppose redefining marriage.”

According to a The Washington Post poll conducted in September, Marylanders who vote regularly are evenly split on the issue, with 49 percent opposed and 48 percent in favor.