Now faced with a top-tier Republican challenger, Colorado Sen. Mark Udall’s campaign on Thursday painted the GOP’s swap of candidates between races as a partisan backroom deal unfit for the state’s independent nature.  

GOP Rep. Cory Gardner dropped his House bid Wednesday in favor of the Senate race, which immediately put Udall’s seat in more danger . Meanwhile, Weld County District Attorney Ken Buck, an unsuccessful Senate candidate in 2010 and a long-shot to defeat Udall, dropped out of the Senate race to run for Gardner’s seat.  

“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.”  

Earlier Thursday, Udall was spotted on the Senate floor huddling with Majority Leader Harry Reid of Nevada, No. 3 Senate Democrat Charles E. Schumer of New York; Democratic Senatorial Campaign Committee Chairman Michael Bennet of Colorado and newly-appointed Sen. John Walsh of Montana. Udall also appeared to get well wishes from Sen. Claire McCaskill, D-Mo., who survived a tough re-election of her own in 2012.  

Republicans had a label of their own for Walsh’s appointment earlier this month to the seat of Democrat Max Baucus: the “Big Sky buy-off.”  

Though Udall is still favored in his re-election bid, the Rothenberg Political Report/Roll Call rating shifted Wednesday from “Safe Democrat” to “Democrat Favored” to reflect Gardner’s announcement.  

Niels Lesniewski contributed to this report.