GOP Senate hopeful Ken Buck’s statement comparing the roots of homosexuality with alcoholism has unleashed a backlash — from doctors to high-dollar Democratic operatives — that threatens to drag down his early momentum 13 days before the election.

Gay-rights advocates who applauded Buck’s sensitivity in handling the first successful hate-crime murder prosecution in the country last year say his statement Sunday on NBC’s “Meet the Press” was shocking.

“I worked with Ken and got to know him over several months, and I found him to be open-minded and thoughtful,” said Fred Sainz, vice president of the Human Rights Campaign, the nation’s largest gay-rights organization. Sainz worked at the Gill Foundation during the hate-crime trial.

“His inartful word choice is at odds with the man I got to know,” he said. “Do I believe that this is in his heart? No. But in many ways that makes it worse.”

Sixteen months ago, as Weld County district attorney, Buck led a prosecution team to convict a man of first-degree murder and a hate crime after he killed a transgender woman, Angie Zapata, in Greeley.

At the time, Buck told reporters that “if someone goes after someone here because of their sexual status, we will come after you with everything we have.”

Asked Sunday by NBC reporter David Gregory if he thought being gay is a choice, Buck said, “You can choose who your partner is.

“I think that birth has an influence over, like alcoholism and some other things,” he said. “But I think that, basically, you have a choice.”

Afterward, Buck said he wasn’t a biologist but thought there was some element of predisposition.

“I haven’t studied the issue,” he said. “But that’s my feeling on the issue. I tried to explain my feelings as best I could.”

Human- and gay-rights groups on Tuesday called on Buck to apologize and retract his statement at a news briefing, saying it promulgates a “1950s-style” mentality that is destructive to gay youths, in particular.

“We ask that he retract these comments and undo any damage done to young people in this state,” said Jason Marsden, executive director of the Matthew Shepard Foundation.

Buck’s campaign — relatively light in public events this week as early voting opens — stood by the statement Tuesday, saying in an e-mail that “Ken has a record of fighting for all Coloradans.”

“We’re happy to talk about jobs, spending, the economy, and the other issues that Coloradans are focused on,” said spokesman Owen Loftus.

Speaking at a news conference, Dr. Mark Thrun said homosexuality was officially stripped from the list of psychiatric diseases in 1973.

“On behalf of my gay friends, on behalf of my gay colleagues and behalf of my gay patients, I have to say that many of us are not distressed about being gay,” Thrun said. “We do not have a treatable disease.”

Even the Gill Foundation — bankrolled by multimillionaire Tim Gill — issued a rare statement saying, “Derogatory public comments about gay people have consequences and our leaders have a responsibility to guard against them.”

Buck’s latest misstep is bad news for his campaign in this ever-tightening U.S. Senate race.

Two polls out this week, one by Fox News and another from Reuters, show Buck’s Democratic opponent, Sen. Michael Bennet, has caught up, and the two are effectively tied within the margin of error.

In August, Buck held a comfortable 9-point margin in a Reuters poll.

Alex Hornaday, vice president of the Colorado Log Cabin Republicans, said that while he disagreed with Buck’s “Meet the Press” statement, he will stand by him.

“I felt like he was fairly inarticulate,” said Hornaday, a Denver lawyer. “Even though I have no grand illusions that he supported my positions on gay rights, he does reflect my position on the economy, national security and the size and scope of government, and that’s what is important to me in this election.”

Buck’s senior campaign adviser, Walt Klein, would say only that he appreciated Hornaday’s words.

“Friends who stand by you are hard to find in the last two weeks of a political campaign,” he said.

Allison Sherry: 303-954-1377 or asherry@denverpost.com