Party competition and signalling to regional constituencies: Evidence from co-sponsorship in Portugal

Portugal's closed list proportional electoral system makes parties dominant players in the determination of candidates. The parties face a trade-off between maintaining party unity and gaining votes by allowing regional candidates to defect from the party line. We analyse this trade-off using co-sponsorship data from 1983-2016. Our analysis shows that there is a significant and positive effect in what regards adjacency (close districts) with respect to co-sponsorship. We also demonstrate that there is no specific effect of government and opposition, mainly because we are looking at the MP level (strategies) independently of the party status or type of party.