Ken Buck and Jane Norton are taking swings at each other in their fight for the Republican Senate nomination, Andrew Romanoff and Michael Bennet are now doing the same for the Democratic Senate nomination, and just about everyone is taking swings at Scott McInnis and Dan Maes.
Unaffiliated voters -- who account for a third of the Colorado electorate -- can affiliate with a party up until August 10 and vote in that party's races. Denver will have thirteen vote centers around the city where they can do so.
For other county rules, check with the Colorado Secretary of State's office.