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DENVER — Congressman Cory Gardner, considered the Colorado Republican Party’s brightest star, has decided to challenge U.S. Sen. Mark Udall for his seat this fall.

Sources confirmed that Gardner, who passed on the senate race months ago, has decided to jump in, a story that was first reported Wednesday by the Denver Post.

He’s expected to make the official announcement in the coming days.

Gardner, R-Yuma, will immediately become the front-runner in a primary field of candidates who have yet to inspire any national groups to devote serious resources to challenging Udall, a Democrat seeking a second term.

Gardner’s about-face is an indication that Udall, who’s raised $5 million and had been thought to be safe this cycle, is increasingly vulnerable to a challenge as concerns mount about the implementation Obamacare.

Gardner starts his campaign with $876,000 cash on hand.

“It is ultimate testimony to the changed political climate in Colorado – and across purple America – that a rising star such as Cory Gardner with an unassailably safe Congressional seat and a reputation for being a bit averse to political risk would decide to stake it all on a Senate race against a name-brand Democrat who until a few months ago was regarded as a virtual re-election lock,” said political analyst Eric Sondermann.

“This puts Colorado squarely in the middle of the national battle for Senate control. That said, it remains my contention that if the Colorado seat shifts, it will not be the 51st Republican seat in the U.S. Senate but rather will be part of an even larger wave.”

Gardner has been an outspoken critic of Obamacare, using his family’s own story of having its insurance plan cancelled and replaced with a more expensive plan — a story that’s been questioned by Democrats.

Buck to run for Gardner’s seat, Hill staying in

His entrance is likely to clear the GOP Senate field.

Ken Buck, heretofore the favorite in the primary field, is planning to abandon his Senate bid and run for Gardner’s 4th Congressional seat.

“We need to replace Mark Udall in the Senate, and I believe Congressman Cory Gardner is in the strongest position to make that happen,” said Buck in a statement. “The Senate race has never been about me but about helping change the direction of the country. I hope to have the opportunity to lead the fight for limited government and fiscal responsibility as a member of the U.S. House of Representatives.”

State Rep. Amy Stephens, R-Monument, served alongside Gardner for several years in the statehouse and is also rumored to be considering ending her campaign.

And state Sen. Owen Hill, R-Colorado Springs, told FOX31 Denver Wednesday that he’s staying in the race — and that he wanted no part of what he called a “backroom deal” orchestrated by the party.

“Cory came to me two weeks ago and tried to get me out of this race,” Hill said. “I’m staying in because these backroom deals are what people hate about politics these days. I’m staying in to take back this senate seat not for the party bosses but for the people.”

Had Gardner jumped into the race last year, it’s likely he’d have had few viable primary challengers.

But at the time, beating Udall looked like a more difficult task.

“Cory Gardner has had the luxury of waiting and picking his time for his big move,” Sondermann said. “The fact that he has decided to do so in 2014 says everything about Republican confidence and Democratic vulnerability in this evolving election year.”

It’s also likely that Colorado Republicans will have a new gubernatorial candidate next week, with former Congressman Bob Beauprez appearing set to enter the GOP field.

Democrats, Republicans react

As news of Gardner’s candidacy broke Wednesday, it didn’t take long for Democrats to take notice.

Note these tweets from the Democratic Senatorial Campaign Committee’s Matt Canter:

“Despite his slick Washington insider demeanor, Cory Gardner is just as extreme as Ken Buck, Amy Stephens and all the others,” added the DSCC’s Justin Barasky.

“Gardner wants to decimate Medicare, slash education and even make common forms of birth control illegal. Gardner’s Washington friends may have helped him in the Republican primary, but won’t be able to hide his reckless and irresponsible Tea Party agenda from mainstream Coloradans.”

Udall’s campaign took a far more measured tone in its response.

“From flood and wildfire recovery efforts; to ensuring that every family has the opportunity to get ahead; to standing up to the NSA and protecting Coloradans’ freedom to be left alone, Mark spends every day working to protect Colorado’s special way of life,” said Chris Harris, Udall’s new campaign communications director who just signed on this week.

“Mark looks forward to debating the important issues that impact our future.”

While Gardner, should he win the nomination to take on Udall, is likely to help Republicans make the race far more competitive, it’s still a risk for a relatively young politician thought to be an up-and-comer with leadership aspirations within the House GOP — and, as Canter points out, a voting record that’s representative of his conservative district but may be a tougher sell statewide.

That said, Republicans reacted to the news gleefully, believing that Udall’s reelection bid may have just gotten tougher.