Republican Ken Buck, who in 2010 lost narrowly to Sen. Michael Bennet (D-Colo.), will seek to challenge Sen. Mark Udall (D-Colo.) in 2014.
A campaign adviser told the Denver Post that Buck has filed the paperwork to run against Udall -- a surprise move for the Weld County district attorney, who some had expected to run for state attorney general instead.
Buck lost to Bennet by less than 2 percent in 2010 -- a strong year for Republicans nationally. But, as with some other GOP candidates that year and in 2012, Buck committed some gaffes that appeared to cost him -- including comparing homosexuality to alcoholism and making a remark about high heels while running in the GOP primary against former lieutenant governor Jane Norton.
Despite those problems, some polls showed Buck leading Bennet in the final weeks of their race.
Udall, first elected in 2008, is not considered a top GOP target in 2014 but still hails from a swing state. A Quinnipiac University poll in June showed him with a 45 percent approval rating and 31 percent disapproval.
Rep. Cory Gardner was seen as the GOP's top potential recruit against Udall, but he passed on the race earlier this year.