Andre Azevedo Alves, political science professor at Lisbon's Catolica University and St Mary's University in London, said the corruption investigation was a "very strong blow" to any PS ambitions.
Analysts agree the PSD is likely to come out on top, but doubt its ability to build enough support to form a stable government.
"Assuming the likelihood that the PSD will not have (enough) votes to form a government without Chega... we may go from one political crisis to another," Alves said.
Waiting for a bus in central Lisbon, Ana Bernardino, 23, vented her concerns about the political outlook.
"It is a political crisis and in my opinion it's a bit frustrating that elections are being held again...I'm a bit afraid."
Persons:
Marcelo Rebelo de Sousa, Antonio Costa, illegalities, Costa, Costa's, Intercampus, Andre Azevedo Alves, Andre Ventura, Luis Montenegro, Alves, Antonio Barroso, Ana Bernardino, Maria Ines Ferreira, Catarina Demony, Miguel Pereira, Patricia Rua, Andrei Khalip, Nick Macfie, Angus MacSwan
Organizations:
Socialist Party, Social Democrats, Lisbon's Catolica University, St Mary's University, PSD, Chega, Liberal Initiative, CDS, Thomson
Locations:
Belem, LISBON, Portugal, London, Lisbon, Europe