Brian Brown, the president of the National Organization for Marriage (NOM), believes President Barack Obama's gay marriage endorsement will cost him votes.

NOM, the nation's most vociferous opponent of gay marriage, has already denounced the president's position, but in new interviews the group is predicting it will help Republican nominee Mitt Romney's bid for the White House.

“I believe that this will lead to President Obama being a one-term president,” Brown said in a conference call, according to US News & World Report. “If you look at key swing states – Ohio, Florida, North Carolina, Iowa – those are all states where you had overwhelming majorities vote to protect marriage, and I think that we are going to focus on making very clear that this is a key distinction obviously.”

Brown went on to describe Obama's “evolution” from supporting civil unions to marriage for gay couples as an “unmasking.”

“Given that President Obama has been working to undermine [gay marriage ban the Defense of Marriage Act], had come out against all of the amendments that would protect marriage, I don't really see this so much as an evolution as an unmasking of what he already believed given his actions,” he said.

In an interview with Fox News, Brown added: “This is a massive mistake on the part of President Obama. And I think it's going to backfire. And it's going to make a clear contrast between President Obama and Governor Romney.”

However, a candidate's position on gay marriage ranked dead last in a recent poll of voters' priorities.