The Utah-based Church of Jesus Christ of Latter-day Saints joined with a group called the Coalition to Protect Marriage to push the constitutional amendment, and one critic claimed Mormons donated millions to the effort, funding a majority of it.
An earlier Salt Lake Tribune review revealed that Utahns donated $3.8 million to both sides of the Prop 8 campaign more than 70 percent of it in support of the measure. Included in that total was at least $134,774 in in-kind contributions from the LDS Church for such things as air fare, lodging and audiovisual production services and equipment.
Romney's donation came on Oct. 14, 2008, just weeks before Californians voted to pass the measure.
"Mitt Romney's funding of a hate-filled campaign designed to drive a wedge between Americans is beyond despicable," Human Rights Campaign President Joe Solmonese said in a statement. "Not only has Romney signed NOM's radical marriage pledge, now we know he's one of the donors that NOM has been so desperate to keep secret all these years."
The HRC provided a copy of a tax filing from NOM that the group said it received from a whistle-blower. The donation came through a PAC based in Alabama, one of a handful of state-based groups used by Romney to fund staff and travel between his presidential runs.
Romney's campaign stood by the donation, arguing it underscores the candidate's belief in marriage.
"[Former Massachusetts] Governor Romney believes marriage is an institution between a man and a woman and his PAC made a donation to a group supporting that view," spokeswoman Andrea Saul said.
LDS Church leaders had actively promoted passage of Prop 8, urging members to get involved in the campaign and at one point directed its followers to campaign for the measure.
Fred Karger, who founded the group Californians Against Hate and is now running his own Republican presidential bid, probed the Mormon influence in the Prop 8 battle and says that LDS faithful provided a majority of the funds for the Yes on 8 campaign.
Karger said Friday that Romney appears to have tried to hide his donation, but Karger isn't surprised he contributed.
"As a member of the Mormon Church, he was obligated to contribute," Karger said. "He got off pretty easy, only $10,000, but he did it in a sneaky way."