Ron Paul is losing the GOP presidential nominating contest because he does not support a federal constitutional amendment that would ban gay marriage, Brian Brown, president of the National Organization for Marriage (NOM), has suggested.

In congratulating Mitt Romney for winning the Nevada caucuses on Saturday, Brown noted that the former Massachusetts governor has signed the group's anti-gay marriage pledge and Paul has not.

“NOM congratulates Mitt Romney on another impressive victory in the Nevada caucuses, just days after his big win in Florida,” Brown said in a statement. “Governor Romney, who has signed NOM's Marriage Pledge, has now won contests in three different regions of the country and has clearly established himself as the leading candidate.”

Candidates who sign NOM's 5-point pledge promise to support a federal constitutional amendment banning gay marriage, defend the Defense of Marriage Act (DOMA) in court, appoint judges and an attorney general who will “respect the original meaning of the Constitution,” appoint a presidential commission to investigate the “harassment of traditional marriage supporters,” and back legislation that would allow a ballot question on the issue for voters of the District of Columbia.

While the 76-year-old Paul has said he is personally opposed to gay marriage, he has also called on the government to “butt out” of the marriage business.

“I want the government out. If you're going to have government under the constitution, the states have a lot more authority than the federal government has to define it. I'd rather see it be outside of government and then we would not be arguing about this,” he told the Des Moines Register's editorial board.

“Only Ron Paul, who has said that civil marriage should be abolished all together, has refused [to sign NOM's pledge],” the group added. “Paul is the only candidate who has failed to win a single primary or caucus. Rick Santorum won in Iowa while Newt Gingrich won in South Carolina.”

NOM earlier released a video attacking Paul's position on the issue.