What was the reason that Congress could not just pass a law to outlaw slavery?

Answer 1

Well, the big reason antebellum was that the pro-slavery senators would have just voted it down (the pro-slavery representatives would have too, but that wasn't a guarantee because the number of representatives depends on population, and the south, where slavery was generally permitted, was being out-populated by the north, where it wasn't -- each state regardless of size gets the same number of senators, though, so as long as there were no more free states than slave states, the senate was the big roadblock in the passage of any such law).

Before 1804, the slave states were a majority. The first 13 states had 8 states where slavery was permitted and 5 where it wasn't. Then Vermont joined as a free state, making it 8 to 6, followed by Tennessee and Kentucky as slave states (10 to 6). In 1799 New York abolished slavery (9 to 7, still favorable to the slave states).

However, the 1803 admission of Ohio and the abolishion of slavery in New Jersey in 1804 made the total 8-9. Suddenly the slave states were in a minority, and the growing movement for general abolition started to look like it might become a real concern to them.

The admission of Louisiana (as a slave state) in 1812 balanced the two factions, and after that the slave states considered it a matter of vital importance that free states not gain a senate majority. Indiana (free) was admitted in 1816, followed by the admission of Mississippi (slave) in 1817. Illinois (free) in 1818, Alabama (slave) in 1819.

And then things got messy. Missouri wanted to join as a slave state, but anti-slavery forces in the House tacked on a rider (almost certainly unconstitutional, in hindsight) that would have admitted it as a slave state with "poison pill" laws regarding slavery that would have resulted in it eventually becoming de facto if not de jure a free state (the law specified that no new slaves could be brought into Missouri, and the children of slaves born after its admission into the Union would become free upon reaching the age of 25). The Senate rejected this amendment, and the whole measure stalled. Also, Massachusetts was trying to split its territory into two states (with the southern part remaining Massachusetts and the northern part becoming Maine). Maine would, like Massachusetts, have been a free state at the outset.

The Missouri Compromise allowed the admission of Maine as a free state and the admission of Missouri as a slave state (without the "poison pill" rider). An additional provision was added that forbade slavery in new states created from the former Louisiana Territory north of the southern border of (most of) the state of Missouri, with the exception of Missouri itself.

This lasted until 1854, with free and slave states being admitted approximately in pairs: Arkansas (slave) and Michigan (free) in 1836 and 1837, Florida and Texas (slave) in 1845 followed by Iowa and Wisconsin (free) in 1846 and 1848.

The admission of California in 1850 (a free state, but with one pro-slavery senator) maintained the balance in senators if not in states.

However, at this point, pretty much all of the Louisiana Territory where slave states could be formed had been "used up" and was now already part of an existing state, while there was still quite a bit left for free states to form. Slavery was permitted in Indian Territory (now most of the state of Oklahoma) but that was, well, Indian Territory and not likely to become a state (and gain the vital senate seats) any time soon; it was also allowed in the sparsely populated New Mexico Territory but that wasn't really an immediately viable candidate for statehood either. However, the relatively fertile remainder of the former Louisiana Territory (where slavery was forbidden) was getting close, especially the portion that had already been separated off as the Kansas Territory, with at least the southern part of the Nebraska territory not far behind.

The pro-slavery states considered this an intolerable situation, and the Kansas-Nebraska Act was passed in 1854, effectively abolishing the Missouri Compromise northern limit on slavery and allowing popular soverignty in the territories petitioning for statehood to determine whether the state would be admitted as a slave or free state. Thousands of partisans (both pro- and anti-slavery) flooded the territories to attempt to swing the vote in their preferred direction. It wasn't all limited to genteel cultured debates, either; look up "Bleeding Kansas" sometime.

Still, as a practical matter, this is what kept Congress from passing a nationwide anti-slavery law. But the admission of Minnesota (1858) and Oregon (1860) as free states, with Kansas looking likely to follow suit (which it did, in 1861) is what prompted most of the pro-slavery states to attempt to secede from the Union to avoid such a law.

It could be argued (and was) that such a law would have been unconstitutional, and there's some merit to this viewpoint, since the Constitution grants certain very specific powers to the Federal government while allowing the states themselves to retain sovereignty in all matters not explicitly reserved to the Federal government. The 13th amendment explicitly forbade slavery in the Constitution itself (a Constitutional amendment, properly made, by definition cannot be unconstitutional) and the 14th amendment explicitly spelled out that no right granted to citizens by the US Constitution could be abridged or revoked by any state law or constitution, which closed down that particular line of argument.