Final answer:
The Somerset case, adjudicated by Lord Mansfield, significantly influenced the abolitionist movement in England by paving the way for the abolition of the slave trade and slavery in the British colonies. The decision further inspired abolitionists to use moral suasion in advocating for immediate emancipation, propelling the public to recognize the inhumanity of slavery.
Step-by-step explanation:
The long-term impact of Lord Mansfield's legal decisions, especially the Somerset case, on the abolitionist movement in England was profound and far-reaching. The Somerset case of 1772, where Lord Mansfield ruled that slavery was unsupported by the law in England, heralded a shift in legal thought and was a significant step towards the eventual abolition of the slave trade within the British Empire. This case, coupled with the moral and religious fervor of evangelical abolitionists, built the momentum for the eventual passage of the Slave Trade Act 1807 and the Slavery Abolition Act 1833 which effectively ended slavery in the British colonies.
Abolitionists like William Lloyd Garrison further capitalized on this moral suasion and the increasing public consciousness of the evils of slavery they had brought to light since the 1830s to champion the cause of immediate emancipation. The abolitionist movement, in conjunction with significant legal decisions, contributed to growing tensions and an ever-strengthening call for social reform, ultimately culminating in a mass awakening against the institution of slavery.
While the focus here is on the movement's development in England and the role of Lord Mansfield's decisions, it also highlights the interconnectedness of the abolitionist movement across the Atlantic, where similar strategies and ideologies were being employed in the United States.