Yes, slavery was protected in states which declared it lawful, but certainly not in states which outlawed slavery, and not necessarily in US territories controlled by Congress.
For example, President Jefferson proposed and got passed a law to abolish slavery in what were then called "the Northwest Territories" -- Ohio through Wisconsin.
Such actions were well within constitutional limits.
But what our Founders did not contemplate, and what all Northerners strongly objected to was the Supreme Court's Dred-Scott decision which, in effect, could make slavery lawful and abolition impossible, even in Northern states.
That decision did much to radicalize Republicans and further disintegrate the few remaining Whigs.
Northerners believed the time had come to push-back against the slavocracy's over-reach for power.