This is not always true. Here in West Virginia unaffiliated voters like me used to be able to vote in either primary. Then the Republicans canned that option. So now when I vote in a primary I am asked if I would like the Democrat or unaffiliated ballot. All the choices from the unaffiliated ballot are on the Democrat ballot.
What would you call almost everyone dropping out and backing Biden right before super Tuesday but a thumb on the scale? Less severe but directly traceable to the DNC, what would you call giving debate questions in advance to one candidate but not the other but a thumb on the scale?
This is just what I know of.