It is. It goes to state voter registration data for actual demographics, if I correctly understand.
Monmouth explained how party affiliation works here:
http://monmouthpoll.blogspot.com/2016/08/a-poll-samples-party-composition.html
They purportedly culled the voter rolls and called a basically even sample 34D/34R/32I. People self-identified 36D/26R/39I.
Most pollsters don’t adjust Party ID. They leave it as is. Those that do, do such to their own peril.