As explained here, members of SAG (Screen Actors Guild) and AFTRA (American Federation of Television and Radio Artists) are now voting on a proposed merger of the two unions. There does not seem to be any particular opposition to the idea of merging. There is, however, a battle over the terms of this merger.
The two unions have very different health plans, different dues tables, different pension plans, different entrance requirements, etc. A number of members feel that in an unnecessary rush to merge, the two unions have not done sufficient research and planning on how all those financial matters will be reconciled.
I have no particular view on any of this…but I don't have to have one since I'm not a member of either. If you'd like to read the "pro" and "con" statements that are being sent to those who do vote, here's a link to a PDF-type copy of it. It's pretty simple if a bit repetitive.