<p>Political parties around the world form election alliances in which they field common candidates or joint party lists. Once such alliances have formed, what prompts them to end? Existing perspectives suggest that alliances end when allied parties diverge ideologically or the alliance no longer provides parties with electoral benefit. We suggest a different rationale: changes in allies’ relative electoral strength. When one ally becomes relatively more electorally popular, it demands a renegotiation of the terms of the alliance. If allies then either misperceive the cost to ending the alliance or willingly discount the short-term costs to reap longer-term gains, then such demands for renegotiation result in the end of electoral cooperation. Empirically, we focus on two puzzling cases of alliance termination in the Indian state of Maharashtra, where changes in allies’ levels of relative support account for the collapse of longstanding election alliances.</p>