Senate Republicans have scored an unexpected recruiting coup, with Rep. Cory Gardner (R) opting to challenge Sen. Mark Udall (D-Colo.) this year, according to a person with knowledge of his plans.

Gardner, who is considered the state's top GOP rising star, previously said he would not run for Senate and would instead seek reelection in November.

He has made no public announcement, but is expected to make his plans official in the coming days. He declined to comment when approached by reporters after votes Wednesday afternoon.

The news was first reported by the Denver Post.

Udall, who is in his first term, did not begin the cycle as a top GOP target. But a poll released earlier this month showed him in a virtual tie with a couple less-well-known candidates vying for the GOP nomination.

The GOP field so far includes 2010 nominee Ken Buck, state Sen. Randy Baumgardner, state Sen. Owen Hill and state Rep. Amy Stephens. Buck lost to Sen. Michael Bennet (D) in 2010, and GOP leaders have little faith in his ability to defeat Udall.

Nationally, the GOP needs to win six seats to win the majority -- a goal that increasingly appears to be within reach. Democrats are defending seven seats in states that went for Mitt Romney in the 2012 presidential election.

Colorado went for President Obama by five points.

Ed O'Keefe contributed to this report.

Updated at 5:20 p.m.: Roll Call reports that Buck will end his Senate campaign and instead run for Gardner's House seat.