In 1788, all but two states were slaveholding. In the 1780s, northern New Jersey and southeastern New York had slave populations as high as much of the South outside the Tidewater plantation areas. The blame for the continuation of slavery after 1788 was not just a Southern one. Even after the admission of the Northwest Territory as free states and the end of slavery in New York, New Jersey, et. al., there was no universal opposition to the existence of slavery in the free states. Remember that abolitionists were widely unpopular in large parts of the free states, particularly those areas Kevin Phillips defines as the non-Yankee North, extending from what is now metropolitan New York City to the southern half of Illinois.
In 1788, 11 out of 13 states were slaveholding; by 1860, 15 out of 34 permitted the institution. In the 72 year period, due to the increased admission of free states and the abolition of slavery in the Northeastern states, slave states went from the majority to the minority.
Thus, "slave states" of 1788 were different from those of 1860.
In the 1780s, northern New Jersey and southeastern New York had slave populations as high as much of the South outside the Tidewater plantation areas. According to the 1790 census, New York and New Jersey had a combined slave population of about 32,500. Virginia had almost 300,000. North and South Carolina had over 100,000 each, as did Maryland. Georgia had 29,000, over 10 times as many as all the New England states combined. Over three quarters of all slaves lived in one quarter of the states. There was no universal opposition to slavery, but opposition in the Northern states was starting to spread.