What I know of slavery is this - at the time of the ratification of the Constitution it WAS legal and the Conbstitution DID NOT provide any basis to make it illegal. The abolitionists all got their noses up due to the part of the DOI (Dec. of Indep) where it states "We hold these truths to be self-evident that all men are created equal". Slavery in and of itself was an onerous thing and the South was trying to figure out how to do away with it. The major problems the slave owners faced was what to do with the slaves once they were emancipated as the slave owners were still legally responsible for their welfare (training for a job, and finding them a job). So the slave owners faced the prospect of becoming bankrupt if they emancipated their slaves at once.
On the other side of the coin, the Abolitionists wanted immediate emancipation for all slaves, yet they had NO PLAN with what to do with them after emancipation. Amazing when you look at how many ex-slaves were available to work during the industrial revolution here in the States, but the Northern States were too busy putting immigrants to work.
The reason for the Southern secession was caused by two things (one North, and one South). On the Southern side, the admission of territories in the west as "Free Soil" meant that the Southern States would no longer maintain parity in Congress, and as a minority faction, all laws inimicable to Southern interests would get ramrodded through over Southern objections. Along with this, the constant movement by the Abolitionists to foment a Slave uprising (remember John Brown?) put the Southern States on the defensive. On the Northern side, the up and coming industrial power base felt it was time for the North to hold the reins of power in Congress and the Executive branches. They financed the Abolitionist movement and got senators and representatives elected who would cultivate their interests (Stevens, Sumner, and a few others plus Abe Lincoln).
The Southern States were also trying to get free trade with other nations and the North, but the North however, didn't want that to happen as it would have hurt the Northern States economically (i.e. tariffs). There were many other factors that figured into the final mix, but this is a breif overview of why the South felt forced to secede.
The war of aggression was launched by Lincoln who stated that "We must preserve the Union". So he called up 75,000 troops to put down "the rebellion". But look up rebellion and secession and their meanings. Lincoln calling it a rebellion was nothing more than a political ploy to get a war. HE got his war. But if you would care to dig a little deeper, you would find that it wasn't about slavery at all, and that is what galls me about Yankees the most. You buy the line about "It was to free the slaves" and it WASN'T. The North went to war to prevent the Southern States from leaving the Union. They forced the will of Washington DC on the Southern States in 1865. AT THAT time the Declaration of Independence, and the Constitution became "nice to know" items but of no real force. It opened up a whole new era of ways to get around the Constitution by legislation. So it was about political power, and freedom of States. The Northern side fought for centralization of political power and the Industrial barons of the North, the South fought for a State's right to determine its own future. Power, money are most commonly the things that people go to war over, and the War of Aggression waged by the North 1860-1865 was NO different.
Such individuals were not slaves and were citizens of this country despite their condition or the servitude to those they were legally bound to for the sake of meeting a contract they had entered into freely or paying off a debt they owed to a creditor.
But let's look at the whole quote: No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, But shall be delivered up on Claim of the Party to whom such Service or Labour may be due.
I can think of one Founding Father that would have been personally effected by this part of the constitution because he had been a runaway apprentice and legally bound to a "Party to whom such Service or Labour may be due" before the Constitution was written, if we are to define that clause as the very basis of proof that The Constitution legalized slavery then by those words Benjamin Franklin was a slave for most of his adult life.
Yeah I'm stretching the logic a bit, but by precisely the same amount required to convert a "Person held to Service or Labour" into slave by the logic of southern democrats.