We argue that bill cosponsorship in Congress represents an institutional arrangement that provides credibility to commitments of support. We predict that if cosponsorship fosters legislative deals, MCs will only rarely back out on their pledges to support a bill if it comes up for a floor vote, and when they do, these choices will reflect strategic calculations. Further, legislators who violate their cosponsorship agreements will face punishment from colleagues, compromising their ability to gain support for their own bills. We explore the causes and effects of MCs' choices to renege on a pledge by voting no on a bill for which they were a cosponsor, focusing on all cosponsorship decisions in the 101st–108th Houses. The results reveal that patterns of reneging and its consequences are consistent with the idea that cosponsorship functions as a commitment mechanism.