Barack Obama has said he personally supports redefining marriage, but critics have accused him of backing the radical change for political purposes.
President Obama, who is facing a presidential election this year, told a US TV channel that “it is important for me to go ahead and affirm that I think same-sex couples should be able to get married”.
Mitt Romney, the man likely to face Mr Obama in November’s elections, said he supports marriage as being between a man and a woman.
British Prime Minister David Cameron has also said he personally supports same-sex marriage. His Government is consulting on redefining marriage – but half a million people have signed a petition opposed to the plan.
Brian Brown, President of the National Organization for Marriage (NOM), said his group would not let President Obama turn marriage “into something political that can be redefined according to presidential whim”.
Mr Brown commented that President Obama is “beholden to gay marriage activists for campaign financing”.
And the NOM leader also said that the President’s remarks had made “the definition of marriage a defining issue in the presidential contest”.
Religious liberty group the Alliance Defense Fund said marriage is not “something that politicians should attempt to redefine for political purposes”.
Earlier this week in the US state of North Carolina voters backed a constitutional amendment which protects the traditional definition of marriage. The support for marriage included significant numbers of Democrat voters.
Since 1998, in every state where citizens have had the opportunity to vote on the definition of marriage, they have backed the definition of marriage as being between one man and one woman.