The party was founded in 1875. During its initial phase the party was heavily influenced by Proudhonism, and rejected revolutionary Marxism. The party suffered constant factional struggles. In 1919 the leftwing of the party broke away (that group would merge with anarcho-syndicalists to form the Portuguese Maximalist Federation, which became the Portuguese Communist Party). In general the Socialist Party lacked mass support, the trade union movement was led by anarchists and was not affiliated with the socialist International Federation of Trade Unions.