A day after Rick Santorum dropped his bid for the White House, the National Organization for Marriage (NOM) has endorsed Mitt Romney for president.

NOM, the nation's most vociferous opponent of gay marriage, previously promoted Santorum's campaign, though the group fell short of endorsing him for president.

“We are proud to endorse Mitt Romney for President,” NOM President Brian Brown said in a statement. “Governor Romney was an early signer of NOM's presidential pledge which represents his commitment to the nation to take specific actions as president to preserve and protect marriage as the union of one man and one woman.”

“Now is the time for all people who recognize the importance of marriage to come together to support a true champion, Mitt Romney, against an incumbent who has done virtually everything in his power to undermine the institution of marriage.”

“President Obama has declared our nation's marriage laws to be unconstitutional and not only has he refused to defend them, his administration is actively working to repeal them in the courts. He's come out against state constitutional amendments defining marriage as the union of one man and one woman. And he has appointed leaders of the same-sex 'marriage' movement as national co-chairs of his reelection campaign. Incredibly, Obama still apparently claims to personally support traditional marriage.”

“With friends like President Obama, the institution of marriage doesn't need enemies,” Brown added.

The group said it was endorsing Romney “to help focus social conservatives on the need to defeat President Obama in November.”

However, many social conservatives opposed to gay rights sounded less than enthused with Romney as the presumed GOP nominee.

“Looks like we're stuck w/#Romney, 1st major #GOP pres. candidate to be pro-homosexual (eg, supports #gay laws, civil unions) #tcot,” tweeted Peter LaBarbera, founder of Americans for Truth About Homosexuality (AFTAH), to his nearly 1,600 followers.

(Related: Rick Santorum's greatest anti-gay moments on campaign trail.)