The next president should break up some big companies

Last week, Hillary Rodham Clinton recognized that the United States has an oligopoly problem. In an op-ed for Quartz, Clinton wrote that many major industries are dominated by a handful of companies and warned that highly consolidated markets enable firms to raise prices, limit choices and stifle competition. “As president, I will take on this fight,” Clinton wrote, sketching out how she would tackle concentrated economic power.

Clinton’s statement is a big deal. ...