Understanding Senate Majority Leader Mitch McConnell’s handling of the impeachment trial of Donald J. Trump is easy. For some people, impeachment is about high Constitutional questions. For others, it’s about the balance of power between Congress and the White House. But for the wily Republican from Kentucky, it’s about something much more parochial: control of the United States Senate. For McConnell, that means ensuring each Republican Senator facing an electoral challenge this year uses his or her moment as a Trump juror to maximum political advantage.