Republican U.S. Senate hopeful Ken Buck called Tea Partyers questioning the authenticity of President Barack Obama’s birth certificate “dumba—s” to a Democratic operative recording his comments without his permission.

On an audio tape obtained by The Denver Post, Buck was caught muttering “will you tell those dumba—s at the Tea Party to stop asking questions about birth certificates while I’m on the camera?” outside a June 11 event in Crowley.

Buck then started laughing as he walked into the event with the Democratic tracker.

“What am I supposed to do?” he asked, rhetorically.

So-called 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.

Buck has positioned himself as the Tea Party candidate, and on Sunday, he said if he had it to do over again, he would not use the negative language.

Still, he said at an appearance in Broomfield, people who obsess about Obama’s birth certificate at campaign events distract from bigger problems.

“I’m not suggesting the language was appropriate,” Buck told The Post and 9News, which also has the audio tape. “But after 16 months of being on the campaign trail, I was tired and frustrated that I can’t get that message through that we are going to go off a cliff if we don’t start dealing with this debt.”

Political “trackers” are an opposition mainstay on a campaign trail. They usually work for the other political party or an opponent. Because they are everywhere — trailing along with the campaigns, working nights and weekends — candidates often become comfortable with them.

The comment is the third in which Buck has been caught speaking bluntly in recent weeks.

At a July 8 rally with South Carolina Sen. Jim DeMint, former Rep. Tom Tancredo said the biggest threat to the United States was President Obama. Tancredo’s comments went viral and grabbed headlines nationwide.

After the event, Buck told a Democratic tracker, again surreptitiously recording him, that he “can’t believe that guy (Tancredo) opens his mouth.”

Last week, tape was released of Buck telling an Independence Institute group July 17 that voters should pick him “because I do not wear high heels,” he said, referring to his primary opponent, former Lt. Gov. Jane Norton. “I have cowboy boots, they have real bull—- on them.”

Buck on Sunday said the slips are products of grueling months on the campaign trail.

“They’ve had a mic on me for 16 months,” he said. “There are times of frustration where I vent.

“In this case, I vented to the wrong person. It wasn’t a public setting. I was talking to an individual.”

The state Democratic party would not disclose the name of the tracker who recorded the comments.

Political scientist Norman Provizer said “all political campaigns should carry duct tape and use it on candidates whenever they’re out of control.”

But, the Metropolitan State College of Denver professor said Buck likely didn’t alienate too many voters.

“I think overall you can make an aggregate judgment, ‘Is this something that will cause everyone to flock from him and search for something else?’ ” he said. “I don’t think that will happen if he handles it correctly.”

Tea Party leader Lu Busse said Buck should have “used a better choice of words” and shouldn’t have lumped all Tea Partyers together.

“There are certainly many of us who have questions about Obama’s documentation and records,” but that isn’t the biggest problem facing the country, said Busse, chairwoman of the 9.12 Project Colorado Coalition.

“We’re looking at what they (candidates) have done in the past and what they say they are going to do in the future.”

Busse said it probably won’t cost him votes.

“We are going to stay focused on issues,” she said, “and not who calls us names.”

Allison Sherry: 303-954-1377

or asherry@denverpost.com