For the record, CNN corrected the mislabeling a few seconds later, but it’s still a fair question to ask how this happened in the first place.
Screen shots showing Gov Northam as a Republican will be all over the internet forever. LIVoters ( low information applies to people who only watch CNN and only read NYT or WaPo) will never believe that he is a Democrat. He is a racist and a baby-killer. To Dems, being a baby-killer is ok but it must not have been polling well outside DC.
It's easy for me to see how a public school educated young assistant producer could just assume Northam was a Republican. I'm sure he was taught that "all Republicans are racist, and all racists are Republican", so it wouldn't even be necessary to fact check.