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.

“I look forward to continuing to serve and represent the wonderful people of Colorado’s Fourth District in Congress.”

Buck’s announcement quashes prospects of a rematch between the Windsor Republican and Bennet, who defeated Buck by a narrow margin to win his first full term in 2010.

The seat isn’t considered competitive, according to national election forecasters at the Cook Political Report and Sabato’s Crystal Ball, who rate Colorado’s Senate contest as solid Democratic. 

As one of only a handful of high-profile Colorado Republicans still in office after two election cycles dominated by the Democrats, Buck would have likely been the frontrunner in a Republican primary.

Republican sources tell Colorado Politics that no other Republicans have been talking about challenging Bennet, who ran for president last year but bowed out of the Democratic primary early after a poor showing in New Hampshire.