The open letter that 47 Senate Republicans sent to Iran’s leadership on Monday warning about making a nuclear agreement with President Obama is forcing Democrats to choose between confronting Tehran and rallying around Mr. Obama as he searches for a diplomatic solution to the nuclear standoff. Democrats say that as concerned as they are about an emerging deal with Iran, Republicans’ extraordinary moves to undermine Mr. Obama’s efforts to reach an agreement are weakening their resolve to cross party lines and challenge their own president. “I think Republicans have made it harder for us to approach this in a careful and bipartisan way,” said Senator Tim Kaine, Democrat of Virginia, who has been a leader in his party on pushing for congressional review of the administration’s policies on war and sanctions, and is a sponsor of a bill to review any removal of congressionally imposed sanctions on […]