Buck, who lost a senate bid to Michael Bennet (D-Colo.) in 2010, will seek to unseat Colorado’s other incumbent senator, Mark Udall (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.