Ken Buck (R) is making his comeback official.
The 2010 Senate candidate, who narrowly lost to now-Sen. Michael Bennet (D-Colo.) after a series of gaffes, emailed supporters Monday evening to announce he'll run against Sen. Mark Udall (D-Colo.).
He then quickly pivots to Udall, hitting him as "one of the most reliable rubber stamps for President Obama's failed policies."
Buck, a former Weld County district attorney, had filed paperwork to run against Udall in early August.
Republicans have struggled to find a top-tier candidate to run against Udall, and Buck threatens to further complicate the party's efforts to expand the Senate map and challenge him in the swing state. Colorado's Republican Party chairman told The Hill last week that Buck would have "challenges" in the race relating to his last run — which featured comments comparing homosexuality to alcoholism and making fun of his female primary opponent for wearing high heels.
Colorado state Sens. Owen Hill (R) and Randy Baumgardner (R) are already in the race, and former Rep. Bob Beauprez (R) is considering a run.