With the single click of a computer mouse, Donald Trump (or one of his assistants, if the ever-shifting White House narrative of events is to be believed) has delivered manna from his social media account to Democrats gearing up for this year’s US midterm elections. The President’s reposting of racist imagery depicting Barack and Michelle Obama as primates in a jungle has brought opprobrium on Trump’s head from politicians across the spectrum. But for Democrats seeking to eke out an advantage over the President that could help them secure victory in November, the racist post, coupled with Trump’s ongoing refusal...