Interest groups today have deviated far from the Framers' expectations of them as factional brokers of fluid societal coalitions. More surprisingly, they have also become something other than the parochial, corrupting special interests that observers of American politics have long critiqued. We argue that many interest groups have evolved into something less parochial and more programmatic and, distinctively, more partisan. To examine this evolution's timing, extent, and consequences, we analyze the largest dataset ever compiled of interest groups' publicly observable positions on congressional legislation, covering virtually every issue area from the mid-1970s to the present. These data illuminate how the onset of modern party competition has encouraged groups to align with parties, taking more extreme positions on bills outside their core issue interests but that serve party goals. When groups take sides, they sclerose once-fluid lobbying coalitions into hardened recurring sides, distort the parties' understanding of the electorate, and paradoxically weaken their own influence over policy outcomes even when their preferred party is in power. American democracy suffers---as do the groups themselves---when interests are no longer special.