On what ground did Stephen Douglas argue that the Missouri Compromise was already superseded?

1. Under the Missouri Compromise the whole Kansas and Nebraska region was closed to slavery. 2. Missouri slaveholders objected to a free territory on their western side. 3. Douglas answered with an argument drawn from the Compromise of 1850. 4. That compromise had left Utah and New Mexico free to decide on slavery for themselves. 5. If territories could decide, Douglas said, the Missouri Compromise had long been superseded. 6. His bill therefore let settlers carry slaves into Kansas and Nebraska and decide the question themselves. 7. The Dred Scott ruling came three years later, in 1857, so it could not be his ground in 1854. 8. The Ordinance of 1787 barred slavery from the northwest and so cut against his case. _Source: An Outline of American History (U.S. Information Agency, 1990, public domain), section THE SPLIT GROWS DEEPER_