Democratic control of the U.S. Senate – already tenuous as a result of retirements and stiff Republican challenges – has become even more perilous as a result of recent events in Colorado. In what amounts to a game of trading places, Weld County District Attorney Ken Buck, a Republican who intended to challenge incumbent Sen. Mark Udall (D-Colo.) in the November election, dropped out of the race to run for the congressional seat currently held by Rep. Cory Gardner (R-Colo.). Gardner, in turn, jumped into the Senate race, providing Colorado Republicans with a significantly better chance of capturing Udall’s seat....