Complex sentence: This potential expansion of slavery was bitterly opposed by Stevens, who argued that not only was slavery morally wrong, it was uneconomical, and that free states had greatly surpassed Virginia (a slave state) in production.
Split into simpler sentences: This potential expansion of slavery was bitterly opposed by Stevens, who attacked the institution of slavery. He argued that not only was it morally wrong, it was uneconomical, and that free states had greatly surpassed Virginia (a slave state) in production.