All the talk about Graham Platner’s Nazi tattoo and Reddit ramblings has overshadowed some fundamental elements of this year’s Maine Senate race. The most crucial of them? This is the first time since 2002 that Republican incumbent Sen. Susan Collins is running for office while the president hails from her own party. Why is that so important? Since Abraham Lincoln took office in 1861, there have been 41 midterm elections, congressional races during non-presidential election years. In 37 of those races, the president’s party has lost seats on Capitol Hill. In midterm races since 1922, the president’s party has given...