While initially EDA was meant to act as a substitute and political front of the banned Communist Party of Greece, it eventually acquired a voice of its own, rather pluralistic and moderate.

This development was more clearly shown at the time of the 1968 split in the ranks of Communist Party of Greece, with almost all former members of EDA joining the faction with Euro-communist, moderate tendencies.

In the 1958 elections it managed to become the leading party of the opposition, an achievement all the more surprising in view of the recent end of the Greek civil war and the consequently prevailing anti-Left politics at the time.

Before the 1963 election, Greece entered a protracted period of political and social unrest, with the assassination of EDA MP Gregoris Lambrakis, providing further inflammation.

An independent judicial inquiry held under public prosecutor Christos Sartzetakis concluded that those responsible for the assassination were far-right extremists linked with rogue elements in the Greek security forces.