DENVER (AP) — Colorado's Senate race has been roiled by the last-minute entry of rising GOP star Rep. Cory Gardner, a move that increases pressure on both Democratic incumbent Mark Udall and his party as it struggles to maintain control of the Senate.

The front-runner in the race, District Attorney Ken Buck, seen by some Republicans as a flawed candidate, said he is withdrawing from the Senate campaign and running for Gardner's soon-to-be vacant congressional seat. The political musical chairs has triggered charges of a back room deal from at least one other contestant in the GOP primary, but also cheered Republicans desperate for a win in this blue-trending swing state.

"He is exactly the kind of young, articulate, substantial Republican who can not only win a statewide election, but can break this 12-year stranglehold Democrats have had on this state," Dick Wadhams, former chairman of the Colorado Republican Party, said of Gardner.

One Republican, State Senator Owen J. Hill, wasn't happy. He said Wednesday that Gardner had called him two weeks ago urging him to leave the race. "This is exactly why Republicans keep losing, because we're cooking these backroom insider deals," Hill said in an interview, vowing to stay in.

Buck had narrowly lost a 2010 challenge to Democratic Sen Michael Bennet. Several Republican operatives thought his gaffes in that race that offended some women and gays could keep him from beating Udall, too. On Wednesday Buck confirmed Gardner's departure and announced he was stepping aside. "We need to replace Mark Udall in the Senate, and I believe Congressman Cory Gardner is in the strongest position to make that happen," he said.

Gardner had been many Republican operatives' first pick for the Udall race, but he turned it down last year as the Senator consistently polled ahead of potential challengers. But since the bungled rollout of the Affordable Care Act, Obama's approval rating in Colorado polls has plummeted and Udall now only narrowly leads his challengers. Gardner did not comment to reporters in the Capitol on Wednesday.

Gardner's switch comes at the last minute. Caucuses that begin the process of choosing candidates for the primary race are scheduled Tuesday. A second path to the primary ballot involves gathering signatures from the state's congressional districts by March 31.

Republicans are expected to easily hold Gardner's House seat.

Republicans believe Gardner is a superior candidate to Buck because he does not have Buck's history of gaffes and is younger than Buck, 55. But Democrats immediately moved to link the two men, who have a lengthy political history in Colorado's rural northeastern corner.

Matt Canter of the Democratic Senate Campaign Committee said Gardner "wants to decimate Medicare, slash education and even make common forms of birth control illegal." Colorado Democratic Party Chairman Rick Palacios released a statement calling Gardner "simply a Ken Buck radical who is neck deep in Washington sleaze."

Gardner represents a vast district that runs from the lonely high plains at the Nebraska border to the prosperous suburbs at the far southern edge of Denver's metropolitan area and down to New Mexico. He has forged close relationships with House leadership and Rep. Paul Ryan, R-Wis., the 2012 vice presidential nominee, during his time in Washington. He sits on the House Energy and Commerce Committee and plays a major role in energy issues.

Last fall, Gardner announced that his family's health insurance had been cancelled because it did not comply with the minimum standards of the Affordable Care Act and that they had to buy a more expensive plan. Outside groups have already aired ads critical of Udall for voting for Obama's health care package.

Operatives in both parties had expected that, if Buck won the primary, he would struggle to gain support from Super PACs and other outside groups. Republicans say they have no such concerns about Gardner, who had $876,000 in his House campaign account at the end of last year. He can use those funds in his Senate race. Udall has $4.7 million on hand.

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Associated Press writers Charles Babington and Donna Cassata in Washington contributed to this report.