Forty-seven Senate Republicans are signaling in an open letter to Iran and the White House that a deal over Tehran’s nuclear program will be at risk once President Obama leaves office. “It has come to our attention while observing your nuclear negotiations with our government that you may not fully understand our constitutional system,” the senators wrote in the letter, which was first reported by Bloomberg. “Thus, we are writing to bring to your attention two features of our Constitution — the power to make binding international agreements and the different character of federal offices — which you should seriously...