Election Day for the U.S. Senate in Colorado is 18 months away, but a Republican sheriff is already planning to run.Weld County District Attorney Ken Buck has filed the papers to challenge Colorado's freshman Democratic Sen. Michael Bennet.Bennet was superintendent of Denver schools before being appointed earlier this year to fill two years remaining on the term of former Democratic Sen. Ken Salazar. Salazar is now the U.S. interior secretary.In recent months, Buck has traveled to all 64 counties in Colorado to meet with activists and political leadership, according to the Greeley Tribune. Buck has served as district attorney since 2004. He also has worked as a prosecutor, adjunct law professor and businessman.The other potential GOP candidates for the seat include Aurora City Councilman Ryan Frazier, who has formed an exploratory committee, and former U.S. Rep. Bob Beauprez, who said earlier this month that hes "still reading the tea leaves" about whether to enter the contest. Buck has a campaign Web site. He hopes to raise about $8 million to challenge Bennet.