Complex sentence: A lifelong socialist, Heller had previously been a heavy financial donor to the "New York Call," the Socialist Party's New York daily newspaper and had been instrumental in funding the purchase of the headquarters building for the Rand School of Social Science.
Split into simpler sentences: A lifelong socialist, Heller had previously been a heavy financial donor to the "New York Call," the Socialist Party's New York daily newspaper. He had been instrumental in funding the purchase of the headquarters building for the Rand School of Social Science.