Michelle Bachmann, Mitt Romney and Rick Santorum, among those competing with Mr Perry for Republican presidential nomination, have also signed the pledge, the group said.

Mr Perry's move was immediately attacked by Democrats and supporters of gay marriage in his home state.

"I'm afraid that this kind of thing is going to be required by the electorate of the Republican Party to win their nomination," said Matt Glazer, executive director of Progress Texas, a lobbying group for liberal causes. "But this pledge will kill him with independent voters."

Mr Glazer said Mr Perry promised in his August 13 speech in South Carolina announcing his candidacy "to make Washington, D.C., as insignificant in our lives as possible".

"He says he wants to make sure that the federal government isn't involved in much, and then he promotes amazingly invasive federal government policies like this," Mr Glazer said.

Mr Perry had previously appeared to endorse the idea that under the Constitution it should be up to individual states to decide marriage issues.

He later backed away from that comment after criticism from Republican activists.

Mr Brown said his group's stance on marriage "is a winning position for a presidential candidate"