This part of the draft Declaration has intrigued me for years. I would especially like to know:
What specific legislative proposals were the basis for Jefferson’s claim that George III “prostituted his negative” by vetoing them, so as to continue the slave trade?
Were these measures debated and/or passed by Parliament, or by colonial legislatures/councils that required George’s approval?
The British crown was involved in the African slave trade from the time of Elizabeth I, then through the Royal Africa Company. On the surface it seems the grossest hypocrisy for Jefferson to overlook the role of American colonists in creating demand for slaves. I think Jefferson saw slavery as an intractable problem created by the crown acting as “pushers”, who both profited from the trade, and stymied efforts to stop it. He omits to recognize that entrenched interests on both sides of the Atlantic would have been lobbying George to do exactly that; recognizing the reality of that broad support undermines his accusation of tyranny. Adams is correct at that time that accusation was overreaching; later during the War it became apt.
What specific legislative proposals were the basis for Jefferson’s claim that George III “prostituted his negative” by vetoing them, so as to continue the slave trade?"
King George issued decrees such as this one:
"upon pain of the highest displeasure, to assent to no law by which the importation of slaves should be in any respect prohibited or obstructed"(Source)
Jefferson was not the only Founder who recognized the King's role in preventing the colonies from abolishing slavery. At the Convention, George Mason opened one of the debates this way:
"Col. MASON. This infernal trafic originated in the avarice of British Merchants. The British Govt. constantly checked the attempts of Virginia to put a stop to it."(Source)
That's why I found Adams' words so curious. I'd bet that the majority of the Founders held the same view.(whether they wrote it or not is another story)
"On the surface it seems the grossest hypocrisy for Jefferson to overlook the role of American colonists in creating demand for slaves."
That's what makes the king's role so profound. As you pointed out, the crown was involved with slavery from Elizabeth and then in the Royal African Company.
If the American colonists had demand for slaves, who created that demand for slaves in the first place? It all goes back to the king.
Furthermore, if the king was vetoing measures, that means the measures were passing. So the colonial legislatures, listening to their constituents, had many more voices for liberty than they had "entrenched interests" for slavery. But the king, in vetoing such measures, was helping create even more entrenchment.