Laws such as the one which was vetoed, referenced above, this was happening all over the colonies in the 1770s. This wasn't a one time thing.
My intended focus was "This isn't a one time thing." Pennsylvania passed laws very similar to this Virginia law raising taxes to put a stop to slave trading, following a campaign(I forget some of the details at the moment) by prominent abolitionist Anthony Benezet and Benjamin Rush. Benezet, as you may know, was promoted by Ben Franklin. Of course, the law was vetoed by the Royalist governor.
As an aside, putting a stop to the slave trade instead of slavery is actually where the abolitionists were at in those days.
Massachusetts did pass an outright abolition of the slave trade around the same time period, in the 1770s, and it was vetoed by the Royal governor. Rhode Island also passed an abolitionist law in the 1770s, written by one of the signers of the Declaration of Independence.
And yes, I did intend "This isn't a one time thing" both in the context of laws passed as well as vetoes received. Most of the laws that I am aware of passed the colonial legislatures - this is what the people wanted. It was the King and the King's men who didn't want it in the pre-revolutionary era. The crown wanted slavery to stay in place.
I have stated that the Democratic party was the slavery party. I expect some Democrat to reply that there was slavery before there was a Democratic party. If that excuse absolves Democrats, then it also absolves the United States.
That's the part I wasn't so sure of but a bit of research shows that you are right. The word "abolition" seems to have been originally used both for the abolition of slavery generally and also for the abolition of the slave trade specifically.
From the OED "Abolition":
[1773 Pennsylvania Gaz. 13 Jan. 4/1 Some Regulations that have taken Place in the Spanish Colonies, which..are certainly worthy our Imitation, in case we should not be so happy as to obtain an entire Abolition of Slavery.]
1785 G. Gregory Ess. Hist. & Moral 320 The general arguments concerning the good policy of slavery and the slave trade... If it can be proved that good policy..condemns the measure under our consideration,..we may reasonably hope for its final abolition.
1788 T. Clarkson (title) Essay on the comparative Efficiency of Regulation or Abolition as applied to the Slave-trade.
1790 G. Washington Diaries IV. 104 He used arguments to show the..impolicy of keeping these people in a state of Slavery; with declarations, however, that he did not wish for more than a gradual abolition.
From the OED "Abolitionist":
1791 Deb. Abolition Slave-trade 46 If some of the circumstances of cruelty were proved, which the Abolitionists have only asserted, [etc.].
1830 Clarkson's Abolition of Afr. Slave-trade by Brit. Parl. II. iii. 89 Many looked upon the abolitionists as monsters.
And here's links to the last two word cited above:
https://archive.org/details/a160ceb5-bca2-4eb2-8435-bc96f6518ae8/mode/2up
https://archive.org/details/ASPC0002373001/page/n7/mode/2up?q=abolition
You may also be interested in this (old) book that has a lot of detail on the subject: https://www.google.com/books/edition/The_Wrong_of_Slavery_the_Right_of_Emanci/CSuWlNgwK4UC
Starting on p. 85 or 86