Last week, Senate Majority Leader Mitch McConnell turned to the members of his caucus with their eyes on the White House, and asked, "You think running for president is hard?" He didn't have to complete the thought, but it would have been: Try running the Senate. The Kentucky Republican has been tested from the start, but the debate over legislation to give Congress a role in reviewing the Obama administration's potential nuclear agreement with Iran is showing just how complex his job can be.