The literature on pre-electoral coalitions (PECs) still relies heavily on comparative statics to gauge whether a PEC is likely to form and/or its effects on government formation. However, less is known about the behavioural dynamics of PECs. The dynamics of the recent 2009 PEC between the Socialist People's Party and the Social Democrats in Denmark are assessed. Elaborating on the signalling hypothesis, it is argued that coalition members over time must show their ability to vote together in parliament in order to establish a credible commitment. Empirical support is found for this take on the signalling thesis. It was also found that the distance between the parties narrows before the formulation of the PEC, and distinct phases in the process are pointed to, which the authors coin 'friendship', 'courting', and 'engagement'. Finally, alternative explanations are assessed and the question of which party of the PEC would have to change voting patterns in the inter-party coordination process in order to become 'office-fit' is addressed. It is concluded that PECs have a systematic effect on the members' parliamentary voting behaviour and that they serve as a preparatory signalling device for opposition parties with office ambitions.