The primary matchup between Senator Edward J. Markey and Representative Joseph P. Kennedy III is still hypothetical, but it’s already getting messy. An apparent social media blunder by Markey’s top campaign hand has angered Kennedy, his team, and supporters — just as the 38-year-old Newton Democrat is weighing a challenge to the 73-year-old Markey. The controversy started late Monday night when a senior adviser to Markey’s campaign retweeted a nasty post: “@EdMarkey, co-author of the green new deal, is a great Senator. @joekennedy should focus on his family’s considerable mental health issues.” Drama on Twitter ensued: A Kennedy supporter called...