The Social Democratic Party of Hungary (Hungarian: Magyarorszagi Szociáldemokrata Párt, MSZDP) is a small social-democratic political party in Hungary.
The party grew in power and influence until the First World War, which resulted in the party fracturing into pro-war and anti-war factions.
The MSZDP fought the election of November 1945, finishing in a strong second place. From 1945-1948, the MSZDP governed in association with the Smallholders' Party. They were placed under increasing pressure to merge with the Hungarian Communist Party. Eventually, the right-wing were forcibly excluded from the MSZDP - and this allowed the party to officially join the Soviet-sponsored Hungarian Working People's Party. With the establishment of the police state, members were increasingly pressured to co-operate with the Communists. The grouping became the largest party in 1947, but by the end of 1948 the MSZDP had ceased to function independently.
The MSZDP re-emerged defiantly in the Hungarian Revolution of 1956. Under the leadership of Anna Kéthly, Gyula Kelemen and József Fischer the MSZDP took a prominent role in Imre Nagy's Provisional Government. For the first time in many years the party newspaper Népszava was published independently. Following the suppression of the Revolution in 1956 and 1957, the MSZDP disappeared again under state repression, and much of the leadership escaped into exile.
More information on the Wikipedia page.