DENVER (AP) — A Republican Senate candidate for Colorado apologized Monday for making disparaging remarks about tea party members who questioned the authenticity of President Barack Obama's birth certificate.

Ken Buck was caught on audio tape expressing his frustration with the so-called birthers, saying: "Will you tell those dumb(expletive deleted) at the tea party to stop asking questions about birth certificates while I'm on the camera?"

His spokesman, Owen Loftus, said the tape was surreptitiously made by a Democratic staffer assigned to follow Buck. It was obtained by the Denver Post and Denver television station KUSA-TV.

Buck, a conservative supported by many tea party members, has said that he believes Obama is a natural-born citizen.

Loftus said Buck regretted using the profanity and that he was frustrated with people asking him about the birth certificate, which takes away the focus from issues like the national debt and Obama's health care reform.

He said Buck had been asked about the birth certificate issue at a campaign event in Pueblo and made the comments before a later event in Crowley on June 11. He said such unguarded comments are bound to happen after long days of campaigning.

"He was frustrated, and he vented to the wrong person on that," Loftus said.

Trackers from the rival party, such as the Democrat at Buck's event, are typical on the campaign trail, and Buck isn't the first candidate to be embarrassed by one. In 2006, former Virginia Sen. George Allen lost his re-election bid when he was filmed referring to an aide working for his Democratic rival as a "macaca," which is regarded as a racial slur. The aide is of Indian descent.

Buck's comments comes after he was recently filmed telling a Republican voter to choose him over GOP opponent Jane Norton "because I do not wear high heels." Norton has turned the comment into a campaign ad and used it as an opportunity to talk about the need for more conservative women in Washington, D.C.

Norton seized on Buck's latest gaffe Monday and told reporters that Buck is a phony who pretends to agree with tea partiers.

"Ken Buck feels differently when he thinks — wrongly, in this case — that no one is listening," Norton said. She also said that Buck "pretends to be a breath of fresh air."

Norton added that she doesn't personally question about Obama's eligibility to be president. "The birther question has been settled," she said.

Birthers have challenged Obama's standing as president by arguing that he was not born in the United States. Hawaii officials have repeatedly confirmed the president's citizenship, and his Hawaiian birth certificate has been made public, along with newspaper birth notices published when he was born in 1961.

Lu Busse, chairwoman of the tea party group 9.12 Project Colorado Coalition, said she didn't like Buck's choice of words. She said, however, that party members who are accustomed to being called names still largely support him.

She said that includes those who question Obama's place of birth and those who aren't concerned about the issue.

She also said the birth certificate issue is part of a bigger question about Obama's presidency that should be addressed in the 2012 presidential election, not this year's Senate primary.