With midterms rapidly approaching, Republicans had appeared to catch a break on the economy. After a year in which the war with Iran sent gas prices soaring and pushed inflation to multi-year highs, both June and July’s Consumer Price Index came in tame. Even grocery prices slipped for the first time in months. But then President Trump returned to his longtime obsession: tariffs. Indeed, last weekend, talks between the White House and Canada collapsed, and Trump immediately slapped 50 percent tariffs on billions of dollars’ worth of Canadian goods, reviving what the Wall Street Journal called “The Dumbest Trade War...