The fractious debate over a possible nuclear deal with Iran escalated on Monday as 47 Republican senators warned Iran against making an agreement with President Obama and the White House accused them of undercutting foreign policy. In an exceedingly rare direct congressional intervention into diplomatic negotiations, the Republicans sent an open letter addressed to “leaders of the Islamic Republic of Iran” declaring that any agreement could be reversed by the next president “with the stroke of a pen.” The letter appeared aimed at unraveling an agreement even as negotiators grow close to reaching it. Mr. Obama, working with leaders of five other world powers, argues that the emerging agreement would be the best way to keep Iran from obtaining a nuclear bomb, while critics from both parties contend that it would be a dangerous charade that would still leave Iran with the opportunity to eventually build weapons […]