President Barack Obama ’s bet on a diplomatic agreement to deter Iran from acquiring a nuclear weapon faces an immediate test at home, where he must overcome the politics of skeptical Republicans as well as some Democrats in Congress. The political struggle—part of the most complex battle of Mr. Obama’s presidency—is already under way as the GOP-controlled Congress aggressively presses for a bigger role in reviewing the nuclear-framework agreement reached last week between Iran, the U.S. and five other nations. Mr. Obama also must navigate the complicated leadership factions in the Middle East against the backdrop of the region’s increasing volatility. And he has to do all this amid a hotly contested presidential contest. “The stakes are huge,” said Robert Einhorn, formerly one of the Obama administration’s top officials in negotiations with Iran. “It’s a major […]