Ron Paul, Newt Gingrich and Herman Cain have been attacked for not signing an anti-gay marriage pledge.

Maggie Gallagher of the National Organization for Marriage (NOM) blasted the three GOP candidates for president in a recent op-ed published in Real Clear Politics.

NOM had asked all presidential candidates to promise to oppose gay marriage earlier this year.

Gallagher also refuted critics who have suggested that Mitt Romney has reversed course on the issue.

(Related: Mitt Romney says he backs gay rights, not gay marriage.)

“Romney's new mailer in Iowa says he opposes same-sex marriage and supports a federal marriage amendment,” Gallagher wrote. “This is not a flip-flop; it's the truth. On gay marriage he's been a rock. Meanwhile, Ron Paul, Newt Gingrich and Herman Cain have all refused to sign a simple National Organization for Marriage pledge promising to fight for a federal marriage amendment, vigorously defend the federal Defense of Marriage Act and appoint Supreme Court justices who will not invent a right to gay marriage in our Constitution.”

(Related: Ron Paul, Gary Johnson refuse to bash gay marriage.)

“And President Obama? He went before the Rev. Rick Warren in 2008 and told voters out loud that he opposed same-sex marriage. Then as president, Obama turned around and refused to defend against court attacks DOMA, which defines marriage as one man and one woman. His own defenders say Obama's not a flip-flopper – he was just lying to the American people to get elected. In other words, unlike Obama, when it comes to marriage, Mitt Romney (like [Rick] Santorum and [Michele] Bachmann) has demonstrated he's a candidate whose word can be trusted, both politically and personally.”