The purpose of this paper was to seek answers to why the Christian Democrats decided to actively contribute to the December agreement as well as its termination. My questions are: • Why did the Christian Democrats decide to join the December agreement? • Why did the Christian Democrats decide to leave the December agreement? I have used an analysis model based on the model Gunnar Sjöblom developed to explain how parties in a multi-party system reach their overall goal of program realization. As a development of Sjöblom's model, I have added an aspect from Karl Loxbo about the importance of the parties' strategic actions over time. I have also used Tommy Möller's description of eleven factors that affect the scope of action for political leaders at national level. I have used a case study design and made a comparison of two different analysis units in time. To chart the course of events and to find causal mechanisms, I have used process tracking. My conclusions are that the Christian Democrats had a primary focus on cohesion of the Alliance as a starting point for entering into the December agreement and that the party largely left the agreement because the new party leader considered it as a barrier to realizing the party's new political orientation. The situation that arose after the Swedish Democrats entered the Riksdag and brought entirely new conditions remains to be solved.