Together with professor Sakis Karagiorgas, a partner from the resistance, he founded a party called Socialist March (Greek: Σοσιαλιστική Πορεία), which he served as spokesman from 1975 to 1979.

He took part in the 1977 general election as a member of the short-lived Socialist March within the Alliance of Progressive and Left-Wing Forces (Greek: Συμμαχία των Αριστερών και Προοδευτικών Δυνάμεων).

In the same year, he was elected member of the parliament and served as minister for the interior in the coalition government of Tzannis Tzannetakis, New Democracy.

In the 2004 general election held in March, Synaspismos narrowly escaped from being excluded from the parliament again, acquiring 3.2 percent at a national level, despite the fact it had formed an alliance ("Syriza") with other minor parties of the Greek left.

Konstantopoulos received criticism from both his party's members and his left allies for the two consecutive failures, and announced that he would retire as president at the next Synaspismos congress.