In recent years, a large-scale survey was conducted among members of Flemish political parties. The collected data were integrated in a book "Wie is nog van de partij", which was released earlier this year.
The dataset is now available for external use via an online public repository, Zenodo. The dataset can be consulted here. Please refer to reference given below.
B. Wauters & N. Van de Voorde (2017). Wie is nog van de partij-dataset [Data set]. Zenodo. http://doi.org/10.5281/zenodo.1026382