When Republican Rep. Cory Gardner announced in May that he would not challenge Sen. Mark Udall, a Colorado Democrat running for re-election this year, it appeared as though Udall had dodged an electoral bullet.

But Gardner has changed his mind.

Reversing his earlier decision, Gardner, considered a rising star among House Republicans, will run for Senate, the Denver Post reported Wednesday. Ken Buck, a Republican district attorney who had announced he would seek his party's nomination for the Senate seat, will instead switch races to vie for Gardner's open House seat, according to the Greeley Tribune.

"Gardner’s Washington friends may have helped him in the Republican primary, but won’t be able to hide his reckless and irresponsible Tea Party agenda from mainstream Coloradans," Matt Canter, deputy executive director for the Democratic Senatorial Campaign Committee, said in response to the news.

A spokesperson for Udall said he "looks forward to debating the important issues that impact our future."

Gardner's office did not respond to a request for comment.

With Gardner challenging Udall, the race will present a prime, unforeseen opportunity for Republicans to pick up a Senate seat. Meanwhile, national Democrats will likely be forced to spend more money on the race than they had planned, and divert attention and resources from other important races.

Udall will likely enter the race favored to win, but not by much. An April survey by Public Policy Polling showed Udall leading Gardner by 10 points in a head-to-head match-up -- but that was before the disastrous rollout of the president's signature health care law this fall. Obamacare, strategists from both parties predict, will be a key issue for voters in the election.