committed, dedicated (esp. BrE), lifelong, staunch (esp. BrE), strong (esp. BrE)
leading
His father was a leading socialist in Germany.

radical, revolutionary
liberal
moderate
democratic

come to power
The socialists came to power in 1981.

Socialist is used with these nouns:
cause, country, credentials, deputy, economy, feminism, idea, ideal, ideology, mayor, movement, party, point of view, propaganda, realism, regime, republic, revolution, society, state, thinking, thought, writer