Question

Under the Constitution, the principle of "federalism" is the reason that power and responsibility are

Answer 1

read the tenth amendment

Answer 2

The answer that's probably sought is "...divided between the Federal and state goverments."

A "federal" system per se is one in which sovereignty (= the ultimate power to decide) remains with the individual states making up the federation, but a central ("federal") government is appointed to wield some of those powers on behalf of the individual states.

The US Constitution set up a peculiar federal system in which it is not entirely clear whether the power that the people pass to the Federal government goes through the states or not. This was a question that was hotly debated before the Civil War, with one side (that of the future secessionists) arguing that it did. That is, the Federal government acquired its power only via the states. The people of each state authorized their state to participate in the Federal government, and the state did, and that's how the Federal government acquires its power. From this point of view, if the people of a given state choose to withdraw their consent for that transfer, they may, and therefore secession (or nullification, et cetera) is all legal and constitutional. It's sort of like saying the people give the state government their power of attorney, and then the state government gives that to the Feds, and the people can revoke that if they choose.

The other point of view is that the Federal government was constructed not by the individual states, but by the people of all the states collectively and directly. (The fact that they were identified at the Constitutional Convention by states was just a matter of convenience.) From this point of view, the Federal government draws its power directly from the people, bypassing the states entirely. What this means is that no one state can withdraw from the compact -- "revoke" the "power of attorney" -- because that necessarily represents only a minority of all the people in the nation. (Of course, states representing a majority of the people in the union can withdraw, because they can just amend the Constitution, or dissolve it, as they see fit.) This was the point of view held by the northern states, and under which secession and nullification were illegal and unconstitutional.

To some extent these issues were settled by the Civil War, which established the empirical fact that the majority of the states would prevent secession by military force, if necessary, so from a practical viewpoint, we might say the principle has been established that the US federal system draws its power directly from the people, bypassing the states.