Gardner to challenge Udall for Senate seat
© Zach Krahmer

Colorado Republican Rep. Cory GardnerCory Scott GardnerDems look to use Moore against GOP McConnell: 'No change of heart' on Roy Moore US trade deficit rises on record imports from China MORE is planning to challenge Sen. Mark UdallMark 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 candidate in a race that previously favored the incumbent.

Multiple sources confirmed to The Hill that Gardner will make the announcement in a few days. The rising GOP star had opted out of the race last year, leaving the GOP with a wide range of candidates but none with the name recognition or fundraising ability to take on Udall. The Denver Post first reported Gardner's decision. 

According to the Weekly Standard, Gardner has made a "tentative deal" with Weld County District Attorney Ken Buck, who will reportedly now drop his Senate bid and run for Gardner's House seat instead, and state Rep. Amy Stephens, who will drop her bid as well.

No word on the next most prominent Republican, state Sen. Owen Hill, who has nabbed some Tea Party support.

Gardner gives the party the chance to take advantage of what they see as an opportunity in Colorado, where polling has shown Udall is vulnerable. Other already-announced Republican candidates have kept his lead down to single digits in multiple polls of the race, and his approval rating remains below 50 percent.

—This piece was updated at 4:45 p.m.