Who wanted to extend slavery into the United States's newly acquired western territory?

Politicians were forced to deal with the issue of slavery and its westward expansion as early as the Missouri Compromise of 1820. The States had previously maintained a shaky balance in the Senate with an equal number of representatives from both Slave and Free States. As Missouri prepared to enter the Union as a Slave State, this tentative balance threatened to come undone. Henry Clay of Kentucky temporarily solved the issue by crafting the Missouri Compromise, bringing Missouri into the Union as a Slave State and, as a balance, Maine entered as a Free State. The Compromise also made future bondage illegal in all areas of the Louisiana Purchase north of the 36°30′ parallel with the exception of Missouri; all future states below this line would become Slave States.  This Compromise solved the immediate problem of slavery in the Louisiana Purchase by sweeping the real issue of slavery under the rug in order to placate both northern and southern politicians. In the years to come, politicians of both northern and southern states would not be so quick so compromise.

Explanation:

c) he felt the constitution clearly gave states this right.