Newt Gingrich has signed a pledge promising to oppose gay marriage.

The National Organization for Marriage (NOM) announced Gingrich, the former speaker of the House, had joined GOP candidates Minnesota Rep. Michele Bachmann, former Massachusetts Governor Mitt Romney, Texas Governor Rick Perry and former Pennsylvania Senator Rick Santorum in signing their pledge.

Former Minnesota Governor Tim Pawlenty also signed but has since dropped out of the race.

“We commend Newt Gingrich for signing NOM's presidential marriage pledge, committing himself to play a leadership role as president to preserve marriage as the union of one man and one woman,” said NOM President Brian Brown. “Mr. Gingrich joins all the other major candidates who have made a similar commitment, save for one – Ron Paul. Now we will embark on an intensive communications program to inform Iowa voters who will stand with them to preserve marriage, and who has abandoned them on marriage.”

Candidates who sign NOM's 5-point pledge promise to support a federal constitutional amendment banning gay marriage, defend the Defense of Marriage Act (DOMA) in court, appoint judges and an attorney general who will “respect the original meaning of the Constitution,” appoint a presidential commission to investigate the “harassment of traditional marriage supporters,” and back legislation that would allow a ballot question on the issue for voters of the District of Columbia

The moves comes just days after Gingrich vowed to defend DOMA and remain faithful to his wife.

Brown also criticized Texas Rep. Ron Paul for not signing his group's pledge.

“Many of Ron Paul's supporters in Iowa believe that he is on their side when it comes to preserving traditional marriage, but he isn't,” Brown said. “While Paul says he personally believes in traditional marriage, he has refused to sign our pledge and, worse, has said that marriage is strictly a private affair and that government has no role in regulating marriage. This is a dangerous position with profound consequences for society.”

“NOM is not going to endorse a candidate in Iowa, but we will be making it clear through online ads, telephone calls and other actions that Newt Gingrich, Rick Santorum, Mitt Romney and Michele Bachmann stand with Iowans on defending traditional marriage, and Ron Paul does not,” he added.

(Related: Ron Paul's Iowa campaign director Mike Heath blamed wrecked economy on gay marriage.)