Why does the United States sign environmental treaties but not ratify them? U.S. presidents have negotiated and signed several environmental treaties that ultimately could not obtain Senate ratification. This article considers two alternative explanations. First, presidents may face divided government and upcoming elections; elections can increase uncertainty regarding ratification, because they upset majorities and change congressional preferences on issues. Such factors may have caused “involuntary” defection from international environmental cooperation. Second, compensation and compromise on enabling legislation could satisfy enough senators and their constituents to allow the legislation's passage. Failure to secure ratification may be a result of the president's overestimating the potential for negotiating a policy package capable of creating sufficient support to obtain Senate ratification. I compare domestic constraints on U.S. participation in three international environmental negotiations—climate change, biodiversity, and chemicals—to assess the alternative explanations. The cases exemplify how domestic institutions affect international environmental cooperation.