David Leonhardt: Sen. Cory Gardner, R-Invisible

President Donald Trump lost Colorado by 5 percentage points in 2016, and he’s on pace to lose it by more this year. His most recent approval rating there was 39%, according to Morning Consult.His unpopularity creates a problem for Cory Gardner, a first-term Republican senator from Colorado, who will be running for reelection this year — which makes Gardner’s behavior during the impeachment trial especially striking. Rather than act as the senator he is, with his own power to sh