He was a prominent leader of the early Swedish Social Democratic Party.
Danielsson retranslated Karl Marx's The Communist Manifesto into Swedish in 1886.
Danielsson faced legal charges of blasphemy for a controversial article published in Hjalmar Branting's Social-Demokraten: the legal process culminated in the conviction of both men.
Imprisoned in 1888, Danielsson celebrated the event by composing a pamphlet on the labour theory of value while serving out his sentence.
[1] During his imprisonment, his paper was managed by his then fiancée and later wife, Elma Danielsson.