Sen. Mark UdallMark Emery UdallDemocratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups Gorsuch's critics, running out of arguments, falsely scream 'sexist' MORE’s (D-Colo.) campaign blasted his new challenger, Rep. Cory GardnerCory Scott GardnerDurbin: Senators to release immigration bill Wednesday GOP senators eager for Romney to join them Gardner: Bipartisan DACA solution possible despite Trump's 's---hole countries' comment MORE (R-Colo.), for what they called a “Centennial State Swap,” reportedly engaging in dealmaking to clear the GOP field.

Gardner surprised onlookers on Wednesday and gave Republicans renewed hope in the race when he decided to challenge Udall, jumping into a four-person GOP primary that quickly narrowed.

Shortly after his announcement, the original front-runner, Weld County District Attorney Ken Buck, dropped out of the race to run for Gardner’s open seat. On Thursday, another Republican competitor, state Sen. Amy Stephens, also exited and endorsed Gardner.

“It’s no surprise that a Washington ideologue like Congressman Cory Gardner worked with party bosses to cook up the ‘Centennial State Swap,’ the shady scheme hatched behind closed doors that enabled him to swoop into the Senate race,” Udall spokesman Chris Harris said in a statement. “Rather than bring Colorado’s independence to Congress, Gardner is bringing Washington-style backroom deals to Colorado.”

State Sen. Owen Hill, who has received some Tea Party backing, and state Sen. Randy Baumgartner both remain in the primary, and Hill accused Gardner of orchestrating an “insider deal.”

“I’m not surprised at all” Gardner’s in, he told The Hill. “A couple of weeks ago, Cory said he wanted me to get out of the race, and I called him out at the time. I said, ‘This is an insider deal with Ken Buck to try to switch races, and this is the exact kind of insider trading by the Republican establishment that keeps losing us races.’

The narrowed field makes it all the more likely that Gardner will make it through the primary relatively unscathed and without any of the baggage a contentious inter-party fight would bring, setting him up with the best shot to take down Udall.

While Udall still remains the favorite, recent surveys showed his approval ratings were falling in the swing state, and Gardner, Republicans feel, is best-poised to take advantage of that. A Quinnipiac University poll earlier this month showed voters were split on whether Udall should get another term.

Facing even weak GOP challengers, he only had a slim lead. In a match-up against Buck, he only took 45 percent to Buck’s 42 percent. Udall led Hill by 5 percent, with 44 percent support, and topped businessman Jaime McMillan by 7 percentage points, with 45 percent support. He lead state Rep. Amy Stephens by just 43 percent to 41 percent.