In his pledge to aggressively expand American oil and gas production, and his framing of that push as a salvation for the nation’s economic and fiscal health, Donald J. Trump is following in the footsteps of decades of Republican politicians. But in a market where domestic oil production is already higher than it has been in 40 years, and natural gas production is at a historic high, those proposals have run up against a major problem: the global economy. As the presumptive Republican nominee spoke in Bismarck, N.D., this week, in the booming Bakken oil fields not far away, oil...
