Airing on HBO March 24, The Senators’ Bargain is the concluding film of “How Democracy Works Now,” a documentary series that brings the viewer behind the scenes of American democracy and shows how change happens at every level of government. The Senators’ Bargain plunges viewers directly into the backstage reality of a remarkable moment — before the 2008 elections, before the bailouts, before Tea Partiers — when a high-stakes bill six years in the making had a chance to change American immigration policy. But deep at the heart of this fast-moving story, below the level of strategy and protocol, we find a moral tale of modern American politics. Ted Kennedy, one of the handful of people who, through his personal efforts had truly changed the face of America, now would be forced to decide: how much did he want this deal, and what was he willing to trade for his greatest legacy.