From the New York Times:
Congress returns from its August recess next week, and we’ve been busy adding new features and information to our Congress API. One of the bigger gaps we’ve filled is with the cosponsorship of bills: we’ve added two new responses to detail the bills cosponsored by an individual member, and all of the cosponsors for a particular bill. So now you can see when a lawmaker adds his or her support to a bill — or when that support is withdrawn. For bill responses, we’ve also added the party breakdown of cosponsors, so you can judge partisan support.