Republican U.S. Senate candidate Jon Keyser’s primary opponents on Wednesday tore into the former state lawmaker – some more gently than others – after it emerged that Keyser had changed his voter registration to unaffiliated for two years despite boasting that he was “a lifelong Republican” at a recent debate.

Keyser made the claim in response to a direct question asked of all five GOP primary candidates at a May 12 debate in southwest Denver moderated by The Colorado Statesman and Fox31 reporter Joe St. George, although his campaign said Wednesday that Keyser had only left the GOP during a period “when Republicans in Congress lost their way on spending.”

“How long have you been a Republican?” The Statesman asked, reading a question submitted by an audience member. (It might have been intended to elicit an explanation from candidate Jack Graham, who has acknowledged that he was registered as a Democrat until early last year.)

“I’m a lifelong Republican and a second-generation Colorado native,” said Keyser, who was the third candidate to answer the question. “I think we need to elect a Republican that’s going to be able to stand up with principle and a backbone to show a good contrast to beat Michael Bennet.”

“It really wasn’t so much a choice as it was a habit,” Graham said, noting he grew up in a Democratic family that believed “Jack Kennedy walked on water.” He maintains that he hasn’t ever supported a Democratic candidate and points out that the last three Republican senators from Colorado all used to be Democrats at one time or another.

Keyser’s answer was similar to those given by the three other candidates. El Paso County Commissioner Darryl Glenn said he’s been a Republican “since ninth grade – I grew up a Reagan conservative.” Colorado Springs businessman and author Robert Blaha answered, “I’ve been a Republican since I was actually born” and called it “a very relevant question.” Former Aurora City Councilman Ryan Frazier said he’s been “a lifelong Republican” and has “only been registered as a Republican.”

But according to the Jefferson County Elections Division, Keyser was registered unaffiliated for a two-year stretch, from October 2006 to October 2008, changing his registration from Republican and then back again.

“The Republican Party in Washington hasn’t always reflected our conservative values,” Keyser campaign spokesman Matt Connelly told The Statesman on Wednesday. “So, for a period of time Jon was registered as an unaffiliated voter when Republicans in Congress lost their way on spending. Jon believes we need conservative leaders who put their principles ahead of any party.”

The candidate’s unaffiliated past was first reported Tuesday by The Colorado Independent, which also unearthed a 2007 Denver Post story that quoted Keyser, then a first-year law student at the University of Denver, calling the distinction between Democrats and Republicans “very blurry.”

The four other Republicans running for the seat were crying foul on Wednesday.

“It’s unfortunate Jon hasn’t been honest with the Republican voters of Colorado,” Frazier told The Statesman. “It’s clear that Jon is in no position to criticize others in the race for not being conservative enough, or a Republican long enough. It’s sad to see someone with such a bright future continuously self destruct by his own words and actions.”

Keyser took Graham to task at another recent debate, charging that changing affiliation meant switching position on a range of issues, including abortion, immigration and gun rights. “That is not the definition of a conservative, and certainly not the definition of a model conservative,” Keyser told Graham on May 17 at the debate, which was sponsored by The Denver Post.

“It comes down to a bigger issue, which is an issue of integrity,” Glenn’s communications director, Jillian Likness, told The Statesman on Wednesday. “When asked a question, such as how long have you been a Republican, while it may be semantics, it certainly speaks to an integrity issue, and we owe the voters of Colorado the candidate with the integrity and a track record of doing the right thing and standing up for the right thing and backing it up with experience.”

She added, “When you promise to do something, or pledge to do something, there should be support there. And if the evidence isn’t there, it speaks to credibility.”

The Blaha campaign got in a humorous dig at two candidates with one remark.

“Jon was not a Republican for two years,” a Blaha campaign spokesman told The Statesman. “That’s longer than Jack has been one. So put them together, and you almost have a lifelong Republican.”

Graham’s campaign manager, Dick Wadhams, kept it brief.

“Poor Jon’s had several tough weeks, and I’m just not going to pile on,” he said.

– ernest@coloradostatesman.com