Complex sentence: The party continued to exist, but was under the constant threat of legal harassment, and was for all intents and purposes an underground organization until 1934 when a massive campaign finally pushed back many of the police's practice of repression, and the communists were released.
Split into simpler sentences: The party continued to exist, but was under the constant threat of legal harassment, and was for all intents and purposes an underground organization. In 1934 a massive campaign pushed back the police's practice of repression, and the communists were released.