Republican Congressman Cory Gardner intends to drop his re-election bid to run for the U.S. Senate seat held by Democrat Mark Udall, lobbing a bombshell that alters Colorado’s political landscape for the November elections.

Shortly after The Denver Post first reported Gardner’s plans, the GOP front-runner in the Senate race, Weld County District Attorney Ken Buck, revealed he was going to run for Gardner’s seat in the 4th Congressional District.

A number of Republicans, including former state GOP chairman Dick Wadhams, hailed Gardner’s move.

“This is a game changer not only for Colorado Republicans in the Senate race, but also it totally changes the entire 2014 election,” Wadhams said Wednesday. “Cory is someone who all Republicans can rally around and can usher in a new generation of statewide leadership to our party and the state of Colorado.”

Gardner, who is viewed as a rising star by the national GOP, has criticized Udall over his support for the Affordable Care Act.

Udall’s campaign said the senator “looks forward to debating the important issues that impact our future.”

“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 Udall’s campaign spokesman Chris Harris.

Denver political consultant Eric Sondermann said the Gardner move “single handedly puts Colorado at the center of the battle for U.S. Senate that Republicans are waging.”

“It’s a statement that Republicans regard 2014 as their potential turnaround year,” Sondermann said. “And Gardner is really going all in at the poker table. It tells us that this year is the year Republicans believe it’s their best shot at winning the Colorado U.S. Senate race.”

State Sen. Owen Hill, a Colorado Springs Republican who also is running for the Senate, accused Gardner and Buck of cutting backroom deals.

Hill said Gardner approached him two weeks ago and asked him to get out of the race, which he declined to do. Hill blasted “insider Republican Party leaders in this state who think they can decide who the candidates are instead of people.”

“That’s not true,” Buck told The Post. “I stepped out of the race because I thought Cory represents the best opportunity to beat Mark Udall. I told him I would support him because it is so important that we beat Mark Udall and move this country in a different direction.”

Buck also said he told Gardner he wouldn’t run for the 4th District if Gardner thought it would be better for his Senate campaign but that the congressman urged him to run for his seat.

The other leading U.S. Senate candidate is state Rep. Amy Stephens, R-Monument, who is close friends with Gardner. She attended his 2011 swearing-in after he was first elected to Congress. She was not immediately available for comment Wednesday.

Buck, Hill Stephens and other GOP Senate candidates participated in The Denver Post’s Senate debate Tuesday night.

The current crop of candidates has been alternately panned for its message and its fundraising.

Gardner has about $875,000 cash on hand in his congressional re-election account, so he would enter the race with ready cash multiple times that of any other candidate.

Some Republicans on social media applauded Gardner’s move.

Former state Rep. B.J. Nikkel of Loveland wrote on her Facebook page, “YES!!!” Thank you, Cory! Republicans now have a viable option for winning!”

Wadhams early last year tried to talk Gardner into running for the Senate, arguing that Udall could be beaten with the right candidate. Wadhams ran the campaign for former GOP Congressman Bob Schaffer, who lost the Senate race to Udall in 2008. Wadhams said he hasn’t talked to Gardner in weeks and has since backed Stephens.

Gardner served in the state legislature before being elected to the U.S. House in 2010 when he defeated then Democratic-incumbent Betsy Markey. In Congress, he sits on the House Committee on Energy and Commerce.

In recent months, Gardner assailed Udall for

a dispute over health insurance policy cancellations in Colorado, and Gardner called on the state’s division of insurance to provide his office with updated cancellation figures.

The updated numbers found that as of mid-January about 335,000 Coloradans had received cancellation notices, though about 92 percent were offered renewal options.

“It’s stunning that Obamacare has led to this many canceled plans, and now it’s coming to the forefront,” Gardner said at the time.

Gardner is a fifth-generation Coloradan. He and his wife, Jaime, live in Yuma with their two children.

Lynn Bartels: 303-954-5327, lbartels@denverpost.com or twitter.com/lynn_bartels