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Buck, who lost a senate bid to Michael BennetMichael BennetOvernight Finance: Scoop – Trump team eyes dramatic spending cuts | Treasury pick survives stormy hearing Mnuchin: Debt limit increase important, unclear on 'clean' hike Live coverage: Senators grill Trump's Treasury pick MORE (D-Colo.) in 2010, will seek to unseat Colorado’s other incumbent senator, Mark UdallMark UdallLive coverage: Tillerson's hearing for State The rise and possible fall of the ‘Card’ in politics Gardner's chief of staff tapped for Senate GOP campaign director 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.