"The prevailing ideas entertained by [Thomas Jefferson] and most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally, and politically." This was the assessment of the collective mind of America's Founding Fathers, as offered by the vice president of the Confederate States of America, Alexander H. Stephens, in March of 1861 in his famous (or, more properly, infamous) "Cornerstone" speech.
Stephens would go on to say that the Jefferson and the Founders had it all wrong. The races, said Stephens, were not equal, and the new Confederate constitution corrected the Founders of the United States on that point.
I find this critique of America's Founders to be very interesting, considering that modern-day critics are so frequently alleging that the Founders were racist, pro-slavery, and more. Here in Alexander H. Stephens we have an unequivocal white supremacist saying that the foundations of the United States were based on racial equality.
In case you think "racial equality" (in describing the views of Jefferson and the Founders) takes it too far, the fault lies with Stephens (not me). It was Stephens who said that Jefferon's ideals (which indict slavery) "rested upon the assumption of the equality of races" -- an assumption Stephens said was an "error."
For more on this disagreement between Alexander H. Stephens and Thomas Jefferson, I would like to invite you to check out my latest article over at my American Revolution & Founding Era blog...
"Alexander Stephens vs Thomas Jefferson"
As to what the Founders themselves thought of slavery, I would encourage you to check out Vindicating The Founders by Thomas G. West. A great book that addresses the Founders' views on race, gender, and much more.