Here are some useful sources of research on Portuguese elections which you might want to explore further.
Direcção-Geral da Administração Interna. This is an official website of the Portuguese government It provides access to Portuguese election results from parliamentary elections, presidential elections, local elections, referendums and European elections in Portugal from approximately 1999 onwards. It is possible to view statistics and tables relating to total results, results by party
ODIHR - OSCE organisation which produces election monitoring reports. These assess the state of democracy and conduct of elections.
Another excellent staerting point for elections research are election studies. These often involve surveys with voters, offering insight into voting behaviours, sources of information and more. GESIS has a catalogue of studies conducted for different nations.
The Portuguese Voting Behavior website has useful information and free access to some papers related to this.
Finally the Portuguese Political Science Association has specialists working on all aspects of politics relating to Portugal.