A person could be aligned to the SKDL through its basic organisations or as member of the "community members" which were the Communist Party of Finland (SKP), the Democratic League of Finnish Women (1944–1990), Academic Socialist Society (1944–1965), Suomen Toverikuntien Liitto (1946–1952), the Socialist Unity Party (SYP) (1946–1955), the Socialist Student League (1965–) and the Democratic Youth League of Finland (1967–1990). In addition to the community members, tens of different nationwide organizations were controlled by the SKDL members, see for example the People's Temperance League.
The supporters of the SKP constantly had a majority in the SKDL, thus it was regarded by many as a communist 'umbrella organisation'. The SKP members often attended two consecutive meetings to decide on the same issues. However, not even socialism was mentioned in the party programme until the late 1960s. The number of communist party members amongst the SKDL MPs constantly raised from 1945 on, even though many prominent left-wing socialists and former social democrats had joined the alliance in the 1940's.
One of the few organized non-SKP forces in SKDL was the Socialist Unity Party (SYP) which was founded mainly by former social democrats in 1946. The small and marginalized SYP left the SKDL in 1955 but most of the socialists inside the SKDL chose not to follow the decision made by the party chair Atos Wirtanen and they remained members of the SKDL through its basic organisations. In the early 1970's a Joint Committee of the SKDL Socialists was formed but it never developed an organisation and remained a loose coalition.
| K. H. Wiik | 1944 |
| Cay Sundström | 1944–1946 |
| J. W. Keto | 1946–1948 |
| Kusti Kulo | 1948–1967 |
| Ele Alenius | 1967–1979 |
| Kalevi Kivistö | 1979–1985 |
| Esko Helle | 1985–1988 |
| Reijo Käkelä | 1988–1990 |
| Tyyne Tuominen | 1944–1949 |
| Yrjö Enne | 1949–1952 |
| Hertta Kuusinen | 1952–1958 |
| Yrjö Enne | 1959–1961 |
| Mauno Tamminen | 1962–1965 |
| Ele Alenius | 1965–1967 |
| Aimo Haapanen | 1967–1977 |
| Jorma Hentilä | 1977–1984 |
| Reijo Käkelä | 1984–1988 |
| Salme Kandolin | 1988– |
Parliamentary elections
| Local elections
|
| Year | Electors | Votes | Share of votes | Candidate |
|---|---|---|---|---|
| 1950 | 67 | 338 035 | 21,4% | Mauno Pekkala |
| 1956 | 56 | 354 575 | 18,7% | Eino Kilpi |
| 1962 | 63 | 451 750 | 20,5% | Paavo Aitio |
| 1968 | 56 | 345 609 | 17,0% | seconded Urho Kekkonen of Centre |
| 1978 | 56 | 445 098 | 18,2% | seconded Urho Kekkonen of Centre |
| 1982 | 32 | 348 359 | 11,0% | Kalevi Kivistö |
| 1988 | 26 | 330 072 286 833 | 10,7% 9,6% | Kalevi Kivistö Liike 88 |