Ben Carson, frontrunner in the struggle to become Republican presidential candidate, said on Wednesday that his support for gay rights initiatives when he was a director at major companies does not clash with his opposition to gay marriage.

But some leaders in the social conservative movement that has backed Carson say his more than 15-year record on the boards of retailer Costco Wholesale and food manufacturer Kellogg could pose a challenge for him as he seeks the nomination.

Carson supported various initiatives at both companies, such as barring discrimination based on gender identity, providing health insurance for employees' domestic partners, and offering more diversity training. Because of such changes the companies now are ranked as some of the best in the United States by lesbian, gay, bisexual and transgender advocates.

"I think he has to explain this," said Tony Perkins, president of the Christian conservative lobbying group Family Research Council. "As he is pursuing the presidency, what he has to make clear is that the board positions should not be reflective of his public policy."

Perkins and other conservative leaders say they fear that some of the changes at American corporations could be used to punish employees with unpopular political views. They worry, for instance, that workers who do not support same-sex marriage could be found in violation of anti-discrimination policies.

David King, a senior lecturer of public policy at Harvard's Kennedy School of Government, said Carson's stance could cost him backing, especially in the early battleground state of Iowa. Those voters, King said, will "be the toughest group for him to try to convince that he can divorce boardroom politics from presidential politics."

FIERY RHETORIC

One of those voters is Lynn Proudfoot, a Republican activist and self-described social conservative in Des Moines, Iowa, who said he has not decided who to support in a party caucus to be held in February. Told of Carson's stance in the boardrooms, Proudfoot said he was surprised and displeased. "I would look disfavorably on that," he said.

To be sure, Proudfoot said other conservative may be more focused on backing Carson because of his fierce opposition to abortion and give him a pass on the boardroom matters.

Carson's boardroom record appears to clash with his fiery rhetoric on homosexuality in televised interviews: he compared same-sex marriage to bestiality in 2013, and implied in March that sex acts among prison inmates show homosexuality is a choice. He has apologized for both statements.

But Carson has said his choices as a director are consistent with his broader claims that he is not anti-gay, only opposed to same-sex marriage: "My general attitude would have been, of course we protect the rights of everybody," he told Reuters in an interview earlier this month.

He echoed those remarks during the debate between Republican candidates in Boulder, Colorado, on Wednesday when asked about his time on the board of Costco. "There is no reason that you can't be perfectly fair to the gay community," he said.

Gregory Angelo, president of the Log Cabin Republicans, which represents gay conservatives, said the corporate initiatives may reflect new social attitudes and may not hold Carson back in primaries. "I certainly don't see him as some fire-breathing homophobe," Angelo said.

MOCKED BY CLINTON

Fellow directors said they do not recall Carson opposing any of the initiatives presented to the board.

"Ben Carson never came into the boardroom with any kind of social policy commentary," said Jeffrey Brotman, who chairs Costco's board. Kellogg director Donald Knauss said he also remembers Carson went along with the policy changes.

Gay rights questions have drawn attention in the 2016 presidential campaign after this year's U.S. Supreme Court ruling legalizing same-sex marriage. Many large U.S. businesses supported the ruling or have taken other policy steps favored by gay activists in recent years, tracking public attitudes.

Leading Democratic candidate Hillary Clinton mocked Carson for his views on same-sex marriage this month during a meeting sponsored by New York gay activist group Human Rights Campaign, saying Carson "says that marriage equality is what caused the fall of the Roman empire."

Last year Costco was named "best perceived brand among LGBT Americans" by YouGov BrandIndex, an online measure of consumer perception. Kellogg and Costco also both ranked highly in recent surveys by Human Rights Campaign, with Kellogg getting a perfect score of "100" and Costco posting a "90".

HRC also gave top scores to technology giant Hewlett-Packard Co during the years it was led by another Republican candidate, Carly Fiorina.

In the interview, Carson acknowledged the stances he took as a director could be an issue for some conservative voters. But he said he has never strayed from his message. "As far as conservatives are concerned, I've made my position clear to them as well. I believe in traditional marriage. But I don't have anything against the gay community."

Carson joined the Kellogg board in 1997 and the Costco board in 1999 and left both boards in May of this year as he began to push for the Republican nomination.

(Editing by Richard Valdmanis and Martin Howell)