Ken Buck (R) is making his comeback official.

The 2010 Senate candidate, who narrowly lost to now-Sen. Michael BennetMichael Farrand BennetHow Congress is preventing a Medicare bankruptcy during COVID-19 Tom Cotton rips NY Times for Chinese scientist op-ed criticizing US coronavirus response Our national forests need protection — and Congress can help MORE (D-Colo.) after a series of gaffes, emailed supporters Monday evening to announce he'll run against Sen. Mark UdallMark Emery UdallThe 10 Senate seats most likely to flip Democratic presidential race comes into sharp focus Democrats will win back the Senate majority in 2020, all thanks to President Trump MORE (D-Colo.).

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"Thank you for the prayers, cards, emails and kind thoughts while I battled the lymphoma cancer found earlier this year.  I am happy to share that I am finished with chemo, have my strength back and my doctors report there is no detectable cancer in my body," he says in an email to supporters. "With that battle behind me and after prayerful consideration, as well as the encouragement of my family, friends, and supporters across Colorado, I have decided to run for the U.S. Senate."

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.