Graham Platner was looking for a cover-up. Nearly 20 years ago, the Maine Democrat drunkenly selected a skull and crossbones design while out on leave with his fellow Marines in Croatia. It wasn’t until several weeks into his campaign for a key U.S. Senate seat that Platner says he discovered that the design was recognized as a Nazi symbol. Rather than wait through multiple sessions of laser removal, Platner called a friend to help him get rid of it. “It was a phone call like: ‘Hey, I have a tattoo. I found out it’s something I don’t want, can you...