U.S. Rep. Ken Buck, the chairman of the Colorado Republican Party, said Thursday night he won't run for Colorado's U.S. Senate seat next year, leaving Democratic U.S. Sen. Michael Bennet without even a rumored Republican challenger in his bid for a third term. Buck, who announced in December that he isn't seeking a second term running the state GOP, said on Twitter that he is "humbled" by the support he's received for a potential Senate bid but intends to continue representing the 4th Congressional District, which covers Greeley, Castle Rock and the Eastern Plains. "I am humbled by the interest and support I have received to pursue a potential Senate bid, however I will not be running for U.S. Senate in Colorado," Buck tweeted.