When diesel reaches $6.57 a gallon in Iowa, the price tag becomes a political reckoning. Republican lawmakers, caught between the harvest calendar and the election calendar, are pressing Donald Trump to restrict diesel exports — a move that would force a choice between two pillars of the party's economic coalition. The tension between agriculture's need for affordable fuel and the oil industry's appetite for global markets reveals how resource scarcity has a way of making old alliances suddenly complicated.