Every six months, the Treasury issues a report declaring that China’s currency is undervalued, draining dollars and jobs out of the U.S. economy. And every six months, the Treasury defers action, saying the Beijing government is not manipulating its currency. But with the prolonged slump in jobs, pressure is mounting among American politicians — as well as those in Europe, Brazil and emerging markets — to challenge China, even if that means imposing import tariffs to protect domestic industries and jobs.