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.) is officially launching his campaign against 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 (D-Colo.), giving Republicans a top-tier recruit that could help them expand the Senate playing field.

"Across Colorado and our four corners are hard working people who want to live our lives our way, not how the government intends," Gardner says in a statement on his new campaign website. "To get real solutions to our nation’s greatest challenges we must replace the leadership vacuum in Washington. That’s why I'm running for the United States Senate. Together, we can bring Colorado values to Washington."

Gardner held an event in Denver on Saturday to publicly declare the run. The Hill reported on Wednesday that he would run for the seat.

His decision to challenge Udall, months after he said he wouldn't, gives Republicans a potentially formidable candidate for the race and could help boost their chances for a Senate takeover.

Udall's poll numbers have been less than stellar, but until now Republicans struggled to land a serious recruit against him and faced a crowded field of B-list candidates.

 Republicans need to pick up six seats for Senate control, and putting Colorado in play could help them do so.

Gardner's campaign will also help clear the field. Two Republican candidates already in the Senate race, 2010 Senate nominee Ken Buck (R) and Amy Stephens (R), plan to drop out and endorse him.