The U.S. Supreme Court will take up the question next month of whether California with its voter-approved, LDS Church-backed Proposition 8 can ban gay marriage, a case that could have widespread repercussions for other states.
Huntsman says it's time conservatives "start to lead again" and push for their states to join the nine along with the District of Columbia that now allow same-sex marriages.
"The party of Lincoln," Huntsman wrote, "should stand with our best tradition of equality and support full civil marriage for all Americans."
Huntsman as recently as January 2012 had said he was against same-sex marriage, though he said states should be able to decide who can wed.
"On marriage, I'm a traditionalist," Huntsman said at the ABC News debate at New Hampshire's St. Anselm College. "I think that ought to be saved for one man and one woman, but I believe that civil unions are fair."
Huntsman's daughter, Abby Huntsman, said Thursday that her father felt the need to make a persuasive argument connecting Republican values with marriage equality.
"I couldn't be more proud of my dad," she said, "for taking a stand on one of the most important issues we face today."
Not that Huntsman has been afraid to speak out against his own party. During his presidential bid, he taunted his competitors by tweeting that he believed in evolution and trusted scientists on climate change. "Call me crazy," he added.
After leaving the race, Huntsman criticized the GOP candidates and even compared the Republican Party to Communists after he was uninvited to a GOP gathering.
His conversion on same-sex marriage isn't likely to earn him any applause from the right flank of the GOP.
"Him coming out [in support of gay marriage] is probably the biggest surprise since Liberace came out," joked Tom McClusky, senior vice president of government affairs at the Family Research Council, referring to the flamboyant musician who repeatedly denied he was gay.
"There are certain principles that define a conservative and one of them is upholding traditional values of life and marriage," McClusky continued. "He can say what he wants, but he's wrong."
Utah Eagle Forum leader Gayle Ruzicka noted that Huntsman had backed an amendment to Utah's Constitution that barred gay marriage and she says Huntsman has backed out of a promise to oppose same-sex unions.
"Gay marriage is not a conservative practice, it's not a Republican practice and Governor Huntsman is neither one of those things," Ruzicka said. "He should do himself a favor and become a Democrat. Those are the principles he espouses and those are the people he associates with."
Democrats, to be sure, were stoked about Huntsman's change of heart. Equality Utah, the state's largest gay-rights organization, honored Huntsman and his wife, Mary Kaye, last year as "beacons of hope" in the fight for equal rights.
Brandie Balken, executive director of the group, said she's not surprised to see Huntsman back marriage equality since he's been an ally on lesbian, gay, bisexual and transgender issues for some time.
"Indeed, we have seen an increase in Republican support for policies providing equal protections under the law for LGBT people," Balken said, noting the growing support among GOP state lawmakers for nondiscrimination protections.
State Sen. Jim Dabakis, who is openly gay and the head of the Utah Democratic Party, said Huntsman's move may help prompt others to come forward with their own support.
"I think this registers nationally because it's giving more and more people, prominent Republicans in politics who have had a quiet opinion about marriage, a right to speak out and say what they have been saying privately for years," Dabakis said.
But backing civil unions likely cost Huntsman votes in his White House bid; the ex-governor was trounced in the New Hampshire primary, where he made his largest push.
Supporting gay marriage could further alienate GOP primary voters should Huntsman try for the presidency again. He hasn't ruled out another bid.
The ex-governor's choice for announcing his new-found support for same-sex marriage was also interesting: The American Conservative magazine, founded in part by ex-presidential candidate Pat Buchanan, and devoted to finding an "intellectual home for serious, independent-minded conservatives."
Huntsman on Thursday also called on his party to refuse to tolerate those within its ranks who "want Hispanic votes but not Hispanic neighbors," and that the GOP needed to lead on reforming drug policies.
"If conservatives come to the table with solutions that put our communities first," Huntsman said, "it will go a long way toward winning elections."
Robert Gehrke contributed to this story.