After former Massachusetts Gov. Mitt Romney won the Nevada caucuses Saturday, the National Organization for Marriage noted that Texas Rep. Ron Paul was the only Republican candidate that hadn't signed its pledge and was also the only one who failed to win a single primary or caucus.

The group is dedicated to fighting same sex marriage and preventing same sex couples from adopting children.

Candidates who signed NOM's pledge vowed to support a federal marriage amendment defining marriage as between only one man and one woman, protect the Defense of Marriage Act in court, appoint conservative judges, and create a presidential commission to investigate harassment of same sex marriage opponents.

"Three of the four candidates remaining candidates in the race have signed the NOM Marriage Pledge," the group wrote in a statement. "Only Ron Paul, who has said that civil marriage should be abolished all together, has refused. 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."

Paul has said he supports the Defense of Marriage Act, and believes that marriage is between one man and one woman. But the libertarian-leaning candidate opposes a federal marriage amendment because it would interfere with a state's right to define marriage for itself.

He is the only current Republican presidential candidate that has not signed NOM's pledge. Paul has also ignored a similar pledge by the Iowa-based Christian conservative group The Family Leader.

NOM has not endorsed any candidate. But the group vowed to fight against Paul, branding him a "radical" whose policies would destroy the institution of marriage.

In 2010, the Southern Poverty Law Center included NOM in its list of anti-gay groups. That same year, the LGBT rights organizations Human Rights Campaign (HRC) and the Courage Campaign launched a website targeting the group, called “NOM Exposed.”

“NOM has become the leading anti-equality force in this country seemingly overnight,” Fred Sainz, vice president of communications for HRC, said in 2010. “They’ve thrived on voters knowing nothing about them or who fuels them.”