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Buck, who lost a senate bid to Michael BennetMichael Farrand BennetGOP eying 'blue slip' break to help Trump fill the courts NFL star claims he was victim of 'abusive conduct' by Las Vegas police Gardner throws support behind DREAM Act MORE (D-Colo.) in 2010, will seek to unseat Colorado’s other incumbent senator, Mark UdallMark UdallDemocratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups Gorsuch's critics, running out of arguments, falsely scream 'sexist' 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.