The fight for control of the Senate just became a bit rockier for Democrats.

Colorado is emerging as a key battleground in the midterm election campaign, with a fast-moving series of events today changing the landscape of the Senate map.

Sen. Mark Udall, a Colorado Democrat seeking a second term, was once thought to be on a glide-path to re-election. But stiff headwinds for Democrats have suddenly turned his seat into a hot commodity.

Rep. Cory Gardner, a Colorado Republican had been aggressively recruited by party leaders to run for the Senate. He had rebuffed the recruiting efforts - until now. Three Republican officials familiar with his decision tell ABC News that he is poised to jump into the race later this week. And shortly after The Denver Post first reported news of his decision today, one of his leading Tea Party challengers, Ken Buck, decided to abandon his Senate bid to run for Gardner's House seat.

"It's a game-changing day," Dick Wadhams, a former chairman of the Colorado Republican Party, told ABC News. "The brutal truth was we didn't have a very strong candidate running. We had a retread who was the front-runner. Now, we have a candidate who Republicans across the board can rally around."

To win control of the Senate, Republicans must pick up six seats. Gardner's decision to run makes Colorado instantly more competitive and offers Republicans another path.

Republicans have been frustrated by their lack of a strong candidate in the state. Buck, who lost the Senate race in 2010, was running again. His candidacy frustrated Republican leaders who viewed him as too extreme to win over swing voters. He told the Greeley Tribune today that he has been in talks with Gardner for about 10 days.

"I was running against Mark Udall because I think this country needed to change directions," Buck told the Tribune. "And I still think the country needs to change directions, and I think Cory gives us the best chance to get that done," Buck told the Tribune.

Gardner, 39, is not well-known across the state, but is seen in Republican circles as one of the party's rising figures.

Udall, who was elected to Congress in 1998, won his first Senate race in 2008. Like other Democrats, his approval ratings have fallen this year in the wake of the health care law and President Obama's flagging popularity.

While Obama carried Colorado in 2008 and 2012, the state is a classic political battleground, with a fierce independent streak running through the electorate. The strength of Udall and Gardner's appeal to those voters will likely determine the outcome of the election and perhaps control of the Senate.