He found that Congress had no right to exclude slavery from any Territory belonging to the United States; that the owners of slaves had equal rights with the owners of other property; that neither Congress nor a territorial authority had any power to exclude slaves from a Territory.

This doctrine was accepted by extremists in the South and was finally embodied in the Dred Scott decision of 1857.
Abolitionists had meantime evolved a precisely contradictory theory.
They asserted that the Constitution gave no warrant for property in man, except as held under state laws; that with this exception freedom was guaranteed to all; that Congress had no more right to make a slave than it had to make a king; and that it was the duty of Congress to maintain freedom in all the Territories.

Extremists expressed the view that all past acts whereby slavery had been extended were unconstitutional and therefore void.

Between these extreme conflicting views was every imaginable grade of opinion.

The prevailing view of opponents of slavery, however, was in harmony with their past conduct and maintained that Congress had complete control over slavery in the Territories.
When the Mexican territory was acquired, Stephen A.Douglas, as the experienced chairman of the Committee on Territories in the Senate, was already developing a theory respecting slavery in the Territories which was destined to play a leading part in the later crusade against slavery.