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Buck, who lost a senate bid to Michael BennetMichael Farrand BennetConservation group says it will only endorse Democrats who support .5T spending plan Lawmakers can't reconcile weakening the SALT cap with progressive goals How Sen. Graham can help fix the labor shortage with commonsense immigration reform MORE (D-Colo.) in 2010, will seek to unseat Colorado’s other incumbent senator, Mark UdallMark Emery UdallKennedy apologizes for calling Haaland a 'whack job' OVERNIGHT ENERGY: Haaland courts moderates during tense confirmation hearing | GOP's Westerman looks to take on Democrats on climate change | White House urges passage of House public lands package Udalls: Haaland criticism motivated 'by something other than her record' MORE (D-Colo.), in 2014.

According to the Denver Post, Buck will launch his campaign and tour the state in early September.

Buck, a district attorney, joins state Sens. Randy Baumgardner and Owen Hill in seeking the Republican nomination.

In 2010, political watchers believed Bennet, who was appointed by then-Gov. Bill Ritter (D) in 2009 to fill the seat left by Ken Salazar (D), who became secretary of the Interior, was vulnerable to a Republican challenger in a year that a wave election gave the GOP a majority in the House.

But Bennet squeaked out a victory over Buck, whose campaign was plagued by controversial comments. Buck believes global warming is a hoax, and at one point appeared to liken homosexuality to alcoholism.